Method and apparatus for handling msga retransmissions during two step random access procedures in wireless communication system

ABSTRACT

The present disclosure relates to a communication method and system for converging a 5th-Generation (5G) communication system for supporting higher data rates beyond a 4th-Generation (4G) system with a technology for the Internet of things (IoT). The present disclosure may be applied to intelligent services based on 5G communication technology and IoT-related technology, such as smart home, smart building, smart city, smart car, connected car, health care, digital education, smart retail, security and safety services. The present disclosure provides a method and apparatus for handling message A retransmission during 2 step random access procedures.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is based on and claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 62/805,058, filedon Feb. 13, 2019, in the United States Patent and Trademark Office, andunder 35 U.S.C. § 119 to Indian Provisional Patent Application SerialNo. 201931009602 (PS), filed on Mar. 12, 2019, in the Indian PatentOffice, the entire disclosure of each of which is incorporated herein byreference.

BACKGROUND 1. Field of the Disclosure

The disclosure relates generally to a wireless communication system and,more particularly, to an apparatus, a method, and a system for handlingmessage A (MSG A) retransmissions during a two-step random accessprocedure in a wireless communication system.

2. Related Art

To meet the demand for wireless data traffic having increased sincedeployment of fourth generation (4G) communication systems, efforts havebeen made to develop an improved fifth generation (5G) or pre-5Gcommunication system. Therefore, a 5G or a pre-5G communication systemis also referred to as a beyond 4G network or a post long term evolution(LTE) system. A 5G communication system is considered to be implementedin higher frequency (mmWave) bands, e.g., 60 GHz bands, in order toaccomplish higher data rates. To decrease propagation loss of radiowaves and increase transmission distance, beamforming, massivemultiple-input multiple-output (MIMO), full dimensional MIMO (FD-MIMO),an array antenna, analog beam forming, and large scale antennatechniques are discussed in 5G communication systems. In addition, in 5Gcommunication systems, development for system network improvement isunder way based on advanced small cells, cloud radio access networks(RANs), ultra-dense networks, device-to-device (D2D) communication,wireless backhaul, a moving network, cooperative communication,coordinated multi-points (CoMP), reception-end interference cancellationand the like. In a 5G system, hybrid frequency shift keying (FSK) andquadrature amplitude modulation (QAM) modulation (FQAM) and slidingwindow superposition coding (SWSC) as an advanced coding modulation(ACM), and filter bank multi carrier (FBMC), non-orthogonal multipleaccess (NOMA), and sparse code multiple access (SCMA) as an advancedaccess technology have been developed.

The Internet, which is a human centered connectivity network wherepeople generate and consume information, is now evolving to the Internetof Things (IoT) where distributed entities, such as things, exchange andprocess information without human intervention. The Internet ofEverything (IoE), which is a combination of IoT technology and big dataprocessing technology through connection with a cloud server, hasemerged. As technology elements, such as sensing technology,wired/wireless communication and network infrastructure, serviceinterface technology, and security technology have been demanded for IoTimplementation, a sensor network, a machine-to-machine (M2M)communication, machine type communication (MTC), and so forth have beenrecently researched. Such an IoT environment may provide intelligentInternet technology services that create a new value to human life bycollecting and analyzing data generated among connected things. The IoTmay be applied to a variety of fields including smart home, smartbuilding, smart city, smart car, connected cars, smart grid, healthcare, smart appliances and advanced medical services through convergenceand combination between existing information technology (IT) and variousindustrial applications.

Accordingly, various attempts have been made to apply 5G communicationsystems to IoT networks. For example, technologies such as a sensornetwork, MTC, and M2M communication may be implemented by beamforming,MIMO, and array antennas. Application of a cloud RAN as theabove-described big data processing technology may also be considered anexample of convergence between 5G technology and IoT technology.

Recently, there is a need to enhance a two-step random access procedurein next generation wireless communication system.

SUMMARY

An aspect of the disclosure provides a communication method and systemfor converging a 5G communication system for supporting higher datarates beyond a 4G communication system.

In accordance with an aspect of the disclosure, a method performed by aterminal in a wireless communication system is provided. The methodincludes storing a medium access control protocol data unit (MAC PDU) ina buffer, wherein the MAC PDU is obtained for a message A (MSG A)transmission; transmitting, to a base station, the MSG A including theMAC PDU; receiving, from the base station, a message B (MSG B)transmission in response to the MSG A, wherein the MSG B includesfallback information; and transmitting, to the base station, an uplinktransmission based on the fallback information, wherein the uplinktransmission includes the MAC PDU.

In accordance with another aspect of the disclosure, a method of a basestation in a wireless communication system is provided. The methodincludes receiving, from a terminal, an MSG A including a MAC PDU,wherein the MAC PDU is obtained by the terminal for the MSG Atransmission and is stored in a buffer of the terminal; transmitting, tothe terminal, a an MSG B in response to the MSG A, wherein the MSG Bincludes fallback information; and receiving, from the terminal, anuplink transmission based on the fallback information, wherein theuplink transmission includes the MAC PDU.

In accordance with another aspect of the disclosure, a terminal isprovided. The terminal includes a transceiver; and a controllerconfigured to store a MAC PDU in a buffer, wherein the MAC PDU isobtained for an MSG A transmission, transmit, to a base station via thetransceiver, the MSG A including the MAC PDU, receive, from the basestation via the transceiver, an MSG B transmission in response to theMSG A, wherein the MSG B includes fallback information, and transmit, tothe base station via the transceiver, an uplink transmission based onthe fallback information, wherein the uplink transmission includes theMAC PDU.

In accordance with another aspect of the disclosure, a base station isprovided. The base station includes a transceiver; and a controllerconfigured to receive, from a terminal via the transceiver, an MSG Atransmission including a MAC PDU, wherein the MAC PDU is obtained by theterminal for the MSG A transmission and is stored in a buffer of theterminal, transmit, to the terminal via the transceiver, an MSG B inresponse to the MSG A, wherein the MSG B includes fallback information,and receive, from the terminal via the transceiver, an uplinktransmission based on the fallback information, wherein the uplinktransmission includes the MAC PDU.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of certainembodiments of the disclosure will be more apparent from the followingdescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a flow-diagram of a two-step random access procedure;

FIG. 2 is an illustration of a structure of a MSG A in two-step randomaccess procedure according to an embodiment;

FIG. 3 is a flow-diagram of a two-step random access procedure accordingto an embodiment;

FIG. 4 is a flow-diagram of a two-step random access procedure accordingto an embodiment;

FIGS. 5A and 5B are flow-diagrams of a two-step random access procedureaccording to an embodiment;

FIGS. 6A and 6B are flow-diagrams of a two-step random access procedureaccording to an embodiment;

FIGS. 7A and 7B are flow-diagrams of a two-step random access procedureaccording to an embodiment;

FIGS. 8A and 8B are flow-diagrams of a two-step random access procedureaccording to an embodiment;

FIGS. 9A and 9B are flow-diagrams of a two-step random access procedureaccording to an embodiment;

FIGS. 10A and 10B are flow-diagrams of a two-step random accessprocedure according to an embodiment;

FIG. 11 is a flow-diagram of a contention based two-step random accessprocedure according to an embodiment;

FIG. 12 is a flowchart of a contention based two-step random accessmethod according to an embodiment;

FIG. 13 is a flowchart of a contention based two-step random accessmethod according to an embodiment;

FIG. 14 is an illustration of a message A (MSGA) reception timingaccording to an embodiment;

FIG. 15 is an illustration of a physical uplink shared channel (PUSCH)resource signaling according to an embodiment;

FIG. 16 is an illustration of a PUSCH resource signaling according to anembodiment;

FIG. 17 is a flowchart of a method of handling a fallback to randomaccess during a random access channel less (RACH-less) handoveraccording to an embodiment;

FIG. 18 is a flowchart of a method of handling a fallback to randomaccess during a RACH-less handover according to an embodiment;

FIG. 19 is an illustration of handling a fallback to random accessduring a RACH-less handover according to an embodiment;

FIG. 20 is a block diagram of a terminal according to an embodiment; and

FIG. 21 is a block diagram of a base station according to an embodiment.

DETAILED DESCRIPTION

The following description with reference to the accompanying drawings isprovided to assist in a comprehensive understanding of the disclosure asdefined by the appended claims and their equivalents. Includes variousspecific details to assist in that understanding but these are intendedto be regarded as merely exemplary. Accordingly, those of ordinary skillin the art will recognize that various changes and modifications of thedisclosure described herein may be made without departing from the scopeand spirit of the disclosure. In addition, descriptions of well-knownfunctions and constructions are omitted for clarity and conciseness.

The words used in the following description and claims are not intendedto be limited to their dictional meanings, but are merely used tofacilitate understanding of the disclosure. Accordingly, it should beapparent to those skilled in the art that the following description ofvarious embodiments of the disclosure is provided for illustrationpurpose only and not for the purpose of limiting the disclosure asdefined by the appended claims and their equivalents.

It is to be understood that the singular forms “a,” “an,” and “the”include plural referents unless the context clearly dictates otherwise.Thus, for example, reference to “a component surface” includes referenceto one or more of such surfaces.

The term “substantially” indicates that the recited characteristic,parameter, or value need not be achieved exactly, but that deviations orvariations, including for example, tolerances, measurement error,measurement accuracy limitations and other factors known to those ofskill in the art, may occur in amounts that do not preclude the effectthe characteristic was intended to provide.

Blocks of a flowchart (or a sequence diagram) and a combination offlowcharts may be represented and executed by computer programinstructions. These computer program instructions may be loaded on aprocessor of a general purpose computer, special purpose computer, orprogrammable data processing equipment. When the loaded programinstructions are executed by the processor, they create a means forcarrying out functions described in the flowchart. Because the computerprogram instructions may be stored in a non-transitory computer readablememory that is usable in a specialized computer or programmable dataprocessing equipment, it is also possible to create articles ofmanufacture that carry out functions described in the flowchart. Becausethe computer program instructions may be loaded on a computer orprogrammable data processing equipment, when executed as processes, thecomputer program instructions may carry out operations of functionsdescribed in the flowchart.

A block of a flowchart may correspond to a module, a segment, or codecontaining one or more executable instructions implementing one or morelogical functions, or may correspond to a part thereof. In some cases,functions described by blocks may be executed in an order different fromthe listed order. For example, two blocks listed in sequence may beexecuted at the same time or executed in reverse order.

In the disclosure, the words “unit”, “module” and the like may refer toa software component or a hardware component, such as, for example, afield-programmable gate array (FPGA) or an application-specificintegrated circuit (ASIC) capable of carrying out a function or anoperation. However, a unit, or the like, is not limited to hardware orsoftware. A unit, or the like, may be configured so as to reside in anaddressable storage medium or to drive one or more processors. Units, orthe like, may refer to software components, object-oriented softwarecomponents, class components, task components, processes, functions,attributes, procedures, subroutines, program code segments, drivers,firmware, microcode, circuits, data, databases, data structures, tables,arrays or variables. A function provided by a component and unit may bea combination of smaller components and units, and may be combined withothers to compose larger components and units. Components and units maybe configured to drive a device or one or more processors in a securemultimedia card.

Terms or definitions necessary to understand the disclosure aredescribed below. These terms are intended to be construed in anon-limiting way.

A base station (BS) is an entity communicating with a user equipment(UE) and may be referred to as a BS, a base transceiver station (BTS), anode B (NB), an evolved NB (eNB), an access point (AP), a 5G NB (5GNB),or a gNB.

A UE is an entity communicating with a BS and may be referred to as aUE, a device, a mobile station (MS), a mobile equipment (ME), or aterminal.

In recent years several broadband wireless technologies have beendeveloped to meet the growing number of broadband subscribers and toprovide more and better applications and services. The 2^(nd) generationwireless communication system has been developed to provide voiceservices while ensuring the mobility of users. The 3^(rd) generationwireless communication system supports not only voice service but alsodata service. In recent years, the 4G wireless communication system hasbeen developed to provide high-speed data service. However, currently,the 4G wireless communication system suffers from a lack of resources tomeet the growing demand for high speed data services. So, the 5Gwireless communication system is being developed to meet the growingdemand for high speed data services, support ultra-reliability and lowlatency applications.

The 5G wireless communication system will be implemented not only inlower frequency bands but also in higher frequency (e.g., mmWave) bands,e.g., 10 GHz to 100 GHz bands, so as to accomplish higher data rates. Tomitigate propagation loss of radio waves and increase transmissiondistance, beamforming, MIMO, FD-MIMO, array antenna, analog beamforming, and large scale antenna techniques are being considered in thedesign of a 5G wireless communication system. In addition, the 5Gwireless communication system is expected to address different use caseshaving quite different requirements in terms of data rate, latency,reliability, mobility etc. However, it is expected that the design ofthe air-interface of the 5G wireless communication system would beflexible enough to serve UEs having quite different capabilitiesdepending on the use case and market segment of the end customer towhich the UE provides service. A few example use cases the 5G wirelesscommunication system wireless system is expected to address is enhancedMobile Broadband (eMBB), massive MTC (m-MTC), ultra-reliable low latencycommunication (URLLC), etc. eMBB requirements such as tens of Gbps datarate, low latency, high mobility and so on address a market segmentrepresenting conventional wireless broadband subscribers needingInternet connectivity everywhere, all the time and on the go. m-MTCrequirements such as very high connection density, infrequent datatransmission, very long battery life, low mobility address and so onaddress a market segment representing the IoT/IoE envisioningconnectivity of billions of devices. URLLC requirements such as very lowlatency, very high reliability and variable mobility and so on address amarket segment representing an industrial automation application, andvehicle-to-vehicle/vehicle-to-infrastructure communication foreseen asone of the enablers for autonomous cars.

In a 5G wireless communication system operating in higher frequency(mmWave) bands, a UE and a gNB communicate with each other usingbeamforming. Beamforming techniques are used to mitigate propagationpath losses and increase propagation distance for communication at ahigher frequency band. Beamforming enhances transmission and receptionperformance using a high-gain antenna. Beamforming may be classifiedinto transmission (TX) beamforming performed in a transmitting end andreception (RX) beamforming performed in a receiving end.

In general, TX beamforming increases directivity by allowing an area, inwhich a signal propagates, to be densely located in a specific directionby using a plurality of antennas. In this case, aggregation of aplurality of antennas may be referred to as an antenna array, where eachantenna included in the antenna array may be referred to as an arrayelement. The antenna array may be configured in various forms such as alinear array, a planar array, etc. The use of TX beamforming results inan increase in directivity of a signal, thereby increasing propagationdistance. Further, since a signal may not be transmitted in a directionother than a directivity direction, signal interference acting onanother receiving end is significantly reduced. The receiving end mayperform beamforming on a RX signal by using an RX antenna array. RXbeamforming increases RX signal strength transmitted in a specificdirection by allowing propagation to be concentrated in a specificdirection, and excludes a signal transmitted in a direction other thanthe specific direction from the RX signal, thereby providing an effectof blocking an interference signal.

By using beamforming, a transmitter may generate a plurality of transmitbeam patterns of different directions. Each of these transmit beampatterns may be referred to as a TX beam. A wireless communicationsystem operating at a high frequency uses a plurality of narrow TX beamsto transmit signals in a cell as each narrow TX beam provides coverageto a part of the cell. The narrower the TX beam, the higher the antennagain and, therefore, the larger the propagation distance of a signaltransmitted using beamforming. A receiver may generate a plurality of RXbeam patterns of different directions. Each of these receive patternsmay be referred to as an RX beam.

A 5G wireless communication system supports a standalone mode ofoperation as well dual connectivity (DC). In DC a multiple Rx/Tx UE maybe configured to utilize resources provided by two different nodes (orNBs) connected via non-ideal backhaul. One node acts as a master node(MN) and the other node acts as a secondary node (SN). The MN and SN areconnected via a network interface and at least the MN is connected tothe core network. New radio (NR) supports a multi-radio accesstechnology (RAT) dual connectivity (MR-DC) operation whereby a UE in aradio resource control connected state (or RRC_CONNECTED) is configuredto utilize radio resources provided by two distinct schedulers, locatedin two different nodes connected via a non-ideal backhaul and providingeither evolved universal mobile telecommunications system (UMTS)terrestrial radio access (E-UTRA) (e.g., if the node is an ng-eNB) or NRaccess (e.g., if the node is a gNB). In NR, for a UE in RRC_CONNECTED,not configured with carrier aggregation (CA)/DC, there is only oneserving cell including the primary cell (PCell). For a UE inRRC_CONNECTED configured with CA/DC the term “serving cells” is used todenote a set of cells including a special cell(s) and all secondarycells (SCells). In NR, the term master cell group (MCG) refers to agroup of serving cells associated with a master node, including thePCell and optionally one or more SCells. In NR, the term secondary cellgroup (SCG) refers to a group of serving cells associated with asecondary node, including the PSCell and optionally one or more SCells.In NR, PCell refers to a serving cell in MCG, operating on a primaryfrequency, in which the UE either performs an initial connectionestablishment procedure or initiates a connection re-establishmentprocedure. In NR, for a UE configured with CA, an Scell is a cellproviding additional radio resources on top of a special cell. PrimarySCG cell (PSCell) refers to a serving cell in SCG in which a UE performsrandom access when performing a reconfiguration with synchronization(sync) procedure. For a DC operation the term SpCell (e.g., specialcell) refers to a PCell of an MCG or a PSCell of an SCG, otherwise theterm special cell refers to the PCell.

In a 5G wireless communication system, a node B (gNB) or a base stationin a cell broadcast synchronization signal and physical broadcastchannel (PBCH) block consists of a primary synchronization signal (PSS)and a secondary synchronization signal (SSS) and system information.System information includes common parameters needed to communicate incell. In a 5G wireless communication system (also referred as nextgeneration radio or NR), system information (SI) is divided into amaster information block (MIB) and a number of system information blocks(SIBs) where: the MIB is always transmitted on the PBCH with aperiodicity of 80 ms, repetitions are made within 80 ms, and the MIBincludes parameters that are needed to acquire an SIB1 from a cell;

the SIB1 is transmitted on a downlink shared channel (DL-SCH) with aperiodicity of 160 ms and variable transmission repetition. The defaulttransmission repetition periodicity of the SIB1 is 20 ms but the actualtransmission repetition periodicity is up to network implementation.SIB1 includes information regarding an availability and scheduling (e.g.mapping of SIBs to an SI message, periodicity, SI-window size) of otherSIBs with an indication whether one or more SIBs are only providedon-demand and, in that case, the configuration needed by the UE toperform the SI request. SIB1 is a cell-specific SIB; andSIBs other than SIB1 are carried in system information (SI) messages,which are transmitted on the DL-SCH. Only SIBs having the sameperiodicity may be mapped to the same SI message.

In a 5G wireless communication system, a physical downlink controlchannel (PDCCH) is used to schedule downlink (DL) transmissions on aphysical downlink shared channel (PDSCH) and schedule uplink (UL)transmissions on a PUSCH, where downlink control information (DCI) onthe PDCCH includes downlink assignments containing at least a modulationand coding format, a resource allocation, and hybrid automatic repeatrequest (HARM) information related to DL-SCH; uplink scheduling grantscontaining at least a modulation and coding format, resource allocation,and hybrid-ARQ information related to UL-SCH. In addition to scheduling,PDCCH may be used for activation and deactivation of a configured PUSCHtransmission with a configured grant; activation and deactivation of aPDSCH semi-persistent transmission; notifying one or more UEs of a slotformat; notifying one or more UEs of a physical resource block(s)(PRB(s)) and orthogonal frequency division multiplexing (OFDM) symbol(s)where the UE may assume no transmission is intended for the UE;transmission of transmission power control (TPC) commands for physicaluplink control channel (PUCCH) and PUSCH; transmission of one or moreTPC commands for sounding reference signal (SRS) transmissions by one ormore UEs; and switching a UE's active bandwidth part; initiating arandom access procedure. A UE monitors a set of PDCCH candidates inconfigured monitoring occasions in one or more configured controlresource sets (CORESETs) according to corresponding search spaceconfigurations. A CORESET consists of a set of PRBs with a time durationof 1 to 3 OFDM symbols. A resource unit's resource element groups (REGs)and control channel elements (CCEs) are defined within a CORESET witheach CCE consisting of a set of REGs. Control channels are formed byaggregation of CCEs. Different code rates for control channels arerealized by aggregating a different number of CCEs. Interleaved andnon-interleaved CCE-to-REG mapping are supported in a CORESET. Polarcoding is used for PDCCH. Each resource element group carrying PDCCHcarries the resource element group's own demodulation reference signal(DMRS). Quadrature phase shift keying (QPSK) modulation is used forPDCCH.

In a 5G wireless communication system, a list of search spaceconfigurations are signaled by a gNB for each configured bandwidth part(BWP) wherein each search configuration is uniquely identified by anidentifier. An identifier of a search space configuration to be used fora specific purpose such as paging reception, SI reception, and randomaccess response reception is explicitly signaled by the gNB. An NRsearch space configuration includes parametersmonitoring-periodicity-PDCCH-slot, monitoring-offset-PDCCH-slot,monitoring-symbols-PDCCH-within-slot and duration. A UE determines PDCCHmonitoring occasion(s) within a slot using the parameters PDCCHmonitoring periodicity (monitoring-periodicity-PDCCH-slot), the PDCCHmonitoring offset (monitoring-offset-PDCCH-slot), and the PDCCHmonitoring pattern (monitoring-symbols-PDCCH-within-slot). PDCCHmonitoring occasions are in slots x to x+duration where the slot withnumber x in a radio frame with number y satisfies Equation (1) below:

(y*(number of slots in a radioframe)+x-monitoring-offset-PDCCH-slot)modulo(monitoring-periodicity-PDCCH-slot)=0  (1)

The starting symbol of a PDCCH monitoring occasion in each slot having aPDCCH monitoring occasion is given bymonitoring-symbols-PDCCH-within-slot. The length (in symbols) of a PDCCHmonitoring occasion is given in the CORESET associated with the searchspace. A search space configuration includes an identifier of a CORESETconfiguration associated with the search space. A list of CORESETconfigurations are signaled by a gNB for each configured BWP whereineach CORESET configuration is uniquely identified by an identifier. Eachradio frame has a 10 ms duration. A radio frame is identified by a radioframe number or a system frame number. Each radio frame includes severalslots wherein the number of slots in a radio frame and a duration ofslots depends on a sub carrier spacing. The number of slots in a radioframe and a duration of slots depends on a radio frame for eachsupported SCS and is pre-defined in NR. Each CORESET configuration isassociated with a list of transmission configuration indicator (TCI)states. One DL RS ID (SSB or CSI RS) is configured per TCI state. Thelist of TCI states corresponding to a CORESET configuration is signaledby a gNB via RRC signaling. One of the TCI states in a TCI state list isactivated and indicated to a UE by a gNB. A TCI state indicates the DLTX beam (DL TX beam is quasi-collocated (QCLed) with SSB/CSI RS of TCIstate) used by a gNB for transmission of PDCCH in the PDCCH monitoringoccasions of a search space.

In a 5G wireless communication system, bandwidth adaptation (BA) issupported. With BA, a receive and transmit bandwidth of a UE need not beas large as the bandwidth of a cell and may be adjusted: the width maybe ordered to change (e.g. to shrink during a period of low activity tosave power); a location may move in the frequency domain (e.g. toincrease scheduling flexibility); and a subcarrier spacing may beordered to change (e.g. to allow different services).

A subset of a total cell bandwidth of a cell is referred to as a BWP. BAis achieved by configuring an RRC connected UE with BWP(s) and informingthe UE which of the configured BWPs is currently an active one. When aBA is configured, a UE only has to monitor a PDCCH on the one activeBWP, i.e., the UE does not have to monitor a PDCCH on the entire DLfrequency of the serving cell. In an RRC connected state, a UE isconfigured with one or more DL and UL BWPs, for each configured servingcell (i.e., PCell or SCell). For an activated serving cell, there isalways one active UL and DL BWP at any point in time. The BWP switchingfor a serving is used to activate an inactive BWP and deactivate anactive BWP. The BWP switching is controlled by a PDCCH indicating adownlink assignment or an uplink grant, by the bwp-InactivityTimer, bythe RRC signaling, or by a medium access control (MAC) entity uponinitiation of a random access procedure. Upon addition of an SpCell oractivation of an SCell, the DL BWP and UL BWP indicated byfirstActiveDownlinkBWP-Id and firstActiveUplinkBWP-Id, respectively, isactive without receiving a PDCCH indicating a downlink assignment or anuplink grant. The active BWP for a serving cell is indicated by eitheran RRC or a PDCCH. For an unpaired spectrum, a DL BWP is paired with aUL BWP, and BWP switching is common for both UL and DL. Upon expirationof a BWP inactivity timer, a UE switches to the active DL BWP to thedefault DL BWP or an initial DL BWP (if a default DL BWP is notconfigured).

In 5G wireless communication system (also referred to as NR), a randomaccess (RA) procedure is used to achieve uplink time synchronization. ARA procedure is used during initial access, handover, an RRC connectionre-establishment procedure, scheduling request transmission, SCGaddition/modification and data or control information transmission in anuplink by a non-synchronized UE in an RRC CONNECTED state.

During a contention based random access (CBRA) procedure (including foursteps), a UE first transmits a random access preamble (also referred toas Msg1) and then waits for a random access response (RAR) or Msg2 in anRAR window corresponding to its random access preamble transmission. AgNB transmits an RAR on a PDSCH addressed to a random access radionetwork temporary identifier (RA-RNTI).

A RA-RNTI identifies a time-frequency resource (also referred as aphysical random access channel (PRACH) occasion or PRACH TX occasion orRACH occasion) in which a random access preamble was detected by a gNB.The maximum size of an RAR-window is one radio frame, i.e., 10 ms. TheRA-RNTI is calculated as follows in Equation (2):

RA-RNTI=1+s_id+14*t_id+14*80*f_id+14*80*8*ul_carrier_id  (2)

In Equation (2) above, s_id is an index of a first OFDM symbol of aPRACH occasion where a UE has transmitted Msg1, i.e., RA preamble;0≤s_id<14, t_id is an index of a first slot of a PRACH occasion(0≤t_id<80), f_id is an index of a PRACH occasion within a slot in afrequency domain (0≤f_id<8), and

ul_carrier_id is a UL carrier used for Msg1 transmission (0 for NUL(normal uplink carrier) and 1 for SUL (supplementary carrier).

Several RARs for various random access preambles detected by a gNB maybe multiplexed in a same RAR MAC protocol data unit (PDU) by the gNB. AnRAR in a MAC PDU corresponds to a UE's random access preambletransmission if it includes random access preamble identifier (RAPID) ofa random access preamble transmitted by the UE. If the RAR correspondingto its random access preamble transmission is not received during theRAR window and the UE has not yet transmitted the random access preamblefor a configurable (configured by the gNB in a RACH configuration)number of times, the UE retransmits the random access preamble. If theRAR corresponding to its random access preamble transmission is receivedand the UE has transmitted a dedicated random access preamble, the RAprocedure is considered successful.

If the UE has transmitted a non-dedicated (i.e., contention based)random access preamble then upon successful reception of an RAR, the UEtransmits Msg3 in a UL grant received in the RAR. The UE generates a newMAC PDU using the data available in logical channels. Msg3 includes amessage such as an RRC connection request, RRC connectionre-establishment request, an RRC handover confirmation, a schedulingrequest, etc. The Msg3 also includes the UE identity (i.e., a cell RNTI(C-RNTI) or serving temporary subscriber identity (S-TMSI) or a randomnumber). After transmitting the Msg3, the UE starts a contentionresolution timer. While the contention resolution timer is running, ifthe UE receives a PDCCH addressed to a C-RNTI included in the Msg3,contention resolution is considered successful, the contentionresolution timer is stopped and the RA procedure is completed. While thecontention resolution timer is running, if the UE receives a contentionresolution MAC CE including the UE's contention resolution identity(first X bits of common control channel (CCCH) service data unit (SDU)transmitted in the Msg3), contention resolution is consideredsuccessful, the contention resolution timer is stopped and the RAprocedure is completed. If the contention resolution timer expires andthe UE has not yet transmitted the random access preamble for aconfigurable number of times, the UE retransmits the random accesspreamble.

In a 5G wireless communication system, a contention free random access(CFRA) procedure is also supported. CFRA is used for scenarios such ashandover where low latency is required, timing advance establishment foran Scell, etc. An ENB or a gNB assigns to a UE non-contention RApreamble in dedicated signaling. The UE transmits the assignednon-contention RA preamble. The ENB or the gNB transmits the RAR on thePDSCH addressed to an RA-RNTI. The RAR conveys an RA preamble identifierand timing alignment information. The RAR may also include a UL grant.The RAR is transmitted in a RAR window similar to a contention based RAprocedure. A contention free RA procedure terminates after receiving theRAR.

In order to reduce the latency of a four-step CBRA procedure, a two-stepRACH procedure is being studied. The two step RACH refers to a procedurewhich can complete a CBRA in two steps.

FIG. 1 is a flow-diagram of a two-step random access procedure.

Referring to FIG. 1, in step 110, UE 101 transmits a random accesspreamble on a PRACH and a payload (i.e., a MAC PDU) on a PUSCH. Therandom access preamble and the payload transmission is also referred asan MsgA. In the second step 120, after the MsgA transmission, the UE 101monitors for a response from the network 103 (i.e., a gNB). The responseis also referred as an MsgB. The MsgB is addressed to MSGB-RNTI, whereMSGB-RNTI is defined as follows in Equation (3).

MSGB-RNTI=1+s_id+14*t_id+14*80*f_id+14*80*8*ul_carrier_id+14*80*8*2  (3)

In the Equation (3) above, s_id is an index of a first OFDM symbol of aPRACH occasion where the UE 101 has transmitted a PRACH preamble;0≤s_id<14, t_id is an index of a first slot of a PRACH occasion(0≤t_id<80), f_id is an index of a PRACH occasion within a slot in afrequency domain (0≤f_id<8), and ul_carrier_id is a UL carrier used forMsg1 transmission (0 for a normal uplink (NUL) carrier and 1 for asupplementary uplink (SUL) carrier.

If a CCCH SDU was transmitted in the MsgA payload, the UE 101 performscontention resolution using the contention resolution information in theMsgB. The contention resolution is successful if the contentionresolution identity received in the MsgB matches the first 48 bits ofthe CCCH SDU transmitted in the MsgA. If the C-RNTI was transmitted inthe MsgA payload, the contention resolution is successful if the UE 101receives the PDCCH addressed to the C-RNTI. If contention resolution issuccessful, the random access procedure is considered successfullycompleted. Instead of contention resolution information corresponding tothe transmitted MsgA, the MsgB may include a fallback informationcorresponding to the random access preamble transmitted in the MsgA. Ifthe fallback information is received, the UE 101 transmits an Msg3 andperforms contention resolution using an Msg4 as in a CBRA procedure. Ifcontention resolution is successful, the random access procedure isconsidered successfully completed. If contention resolution fails uponfallback (i.e., upon transmitting the Msg3), the UE 101 goes back to thefirst step 110 for MsgA transmission, i.e., the UE 101 transmits arandom access preamble on a PRACH and a payload (i.e., the MAC PDU) onthe PUSCH. If the window in which the UE 101 monitors for a networkresponse after transmitting the MsgA expires and the UE 101 has notreceived the MsgB including contention resolution information orfallback information as described above, the UE 101 goes back to thefirst step 110 and transmits the MsgA. If the random access procedure isnot successfully completed even after transmitting the MsgA aconfigurable number of times, the UE 101 fallbacks to a four step RACHprocedure wherein the UE 103 only transmits the PRACH preamble (i.e.,the Msg1).

The MsgA payload may include one or more of a CCCH SDU, a dedicatedcontrol channel (DCCH) SDU, a dedicated traffic channel (DTCH) SDU, abuffer status report (BSR) MAC control element (CE), a power headroomreport (PHR) a MAC CE, synchronization signal block (SSB) information, aC-RNTI MAC CE, or padding. The MsgA may include a UE identifier (ID)(e.g. a random ID, an S-TMSI, a C-RNTI, a resume ID, etc.) along with apreamble in the first step 110. The UE ID may be included in the MAC PDUof the MsgA. The UE ID such as a C-RNTI may be carried in a MAC CEwherein the MAC CE is included in the MAC PDU. Other UE IDs (such as arandom ID, an S-TMSI, a C-RNTI, a resume ID, etc.) may be carried in aCCCH SDU. The UE ID may be one of a random ID, an S-TMSI, a C-RNTI, aresume ID, an IMSI, an idle mode ID, an inactive mode ID, etc. The UE IDmay be different in different scenarios in which the UE 101 performs theRA procedure.

When the UE 101 performs an RA after power on (before the UE 101 isattached to the network 103), then the UE ID is the random ID. When theUE 101 performs an RA in an idle state after the UE 101 is attached tothe network 103, the UE ID is an S-TMSI. If the UE 101 has an assignedC-RNTI (e.g. the UE 101 is in a connected state), the UE ID is a C-RNTI.In case the UE 101 is in an inactive state, the UE ID is a resume ID. Inaddition to the UE ID, some additional control information may be sentin the MsgA. The control information may be included in the MAC PDU ofthe MsgA. The control information may include one or more of aconnection request indication, a connection resume request indication,an SI request indication, a buffer status indication, beam information(e.g. one or more DL TX beam ID(s) or SSB ID(s)), beam failure recoveryindication/information, a data indicator, a cell/BS/TRP(transmission andreception point) switching indication, a connection re-establishmentindication, a reconfiguration complete or a handover complete message,etc.

FIG. 2 is an illustration of a structure of a message A (or MsgA) in atwo-step random access procedure according to an embodiment.

Referring to FIG. 2, in two-step CFRA, a gNB assigns to a UE a dedicatedrandom access preamble (s) and a PUSCH resource(s) for MsgAtransmission. RACH occasion(s) (or, RO(s)) to be used for preambletransmission may also be indicated. The UE transmits a random accesspreamble 215 on a PRACH occasion 210 and a payload 225 on a PUSCHresource 220 using the contention free random access resources (i.e., adedicated preamble/PUSCH resource/RO). After MsgA transmission, the UEmonitors for a response from the network (i.e., the gNB) within aconfigured window.

2 Step CF RACH: MsgA Payload Handling Upon RA Procedure Completion

FIG. 3 and FIG. 4 are flow-diagrams of a two-step random accessprocedure according to embodiments.

Referring to FIGS. 3 and 4, in a case of two step CF RACH, a gNB 303signals contention free random access resources (i.e., dedicatedpreamble(s)/RACH occasion(s) and PUSCH resource(s)) to a UE 301 in steps310 and 410. The UE 301 transmits a PRACH preamble in a PRACH occasionand an msgA payload (or an msgA MAC PDU) in a PUSCH resource/occasionusing the assigned contention free random access resources in steps 320,330, 420, and 430, respectively.

If the gNB 303 receives the msgA (i.e., both the PRACH preamble and themsgA payload), the gNB 303 transmits an msgB including a RAPID, a TA anda UL grant in step 340. If the RAPID in the received msgB matches theRAPID of the PRACH preamble transmitted by the UE 301, the random accessprocedure is completed. This procedure is shown in FIG. 3. Alternately,if the gNB 303 receives the msgA (i.e., both the PRACH preamble and themsgA payload), the gNB 303 transmits a PDCCH addressed to a C-RNTI.Contention free random access is performed in an RRC connected statewhere the UE 301 has an assigned C-RNTI. The gNB 303 can identify the UE301 based on the received PRACH preamble and, therefore, can transmitthe PDCCH addressed to the C-RNTI. If the UE 301 receives the PDCCHaddressed to the C-RNTI, the random access procedure is completed. Uponcompletion of the random access procedure, buffers and/or HARQ buffersused for the MsgA are discarded.

If the gNB 303 receives only the PRACH preamble, the gNB 303 transmitsthe msgB including a RAPID, a TA and a UL grant in step 440. If theRAPID in the received msgB matches the RAPID of the PRACH preambletransmitted by the UE 301, the random access procedure is completed.This is shown in FIG. 4. Alternately, if the gNB 303 receives the msgA(i.e., both the PRACH preamble and the msgA payload), the gNB 303transmits a PDCCH addressed to a C-RNTI. Contention free random accessis performed in an RRC connected state where the UE 301 has an assignedC-RNTI. The gNB 303 may identify the UE 301 based on the received PRACHpreamble and, therefore, may also transmit the PDCCH addressed to theC-RNTI. If the UE 301 receives the PDCCH addressed to the C-RNTI, therandom access procedure is completed. Upon completion of the randomaccess procedure, buffers and/or HARQ buffers used for the MsgA arediscarded.

In this procedure, the msgA payload is lost if the msgA payload is notreceived by the gNB 303. In order to avoid msgA payload loss, the UE 301may store the msgA payload. Upon completion of the random accessprocedure, the UE 301 may retransmit the msgA payload or discard themsgA payload based on information received in the msgB corresponding tothe transmitted msgA. In the disclosure, several methods are provided toenable the UE 301 to identify whether to retransmit the msgA payload ordiscard the msgA payload upon completion of the two step CF RACHprocedure.

FIGS. 5A and 5B are flow-diagrams of a two-step random access procedureaccording to an embodiment.

Referring to FIGS. 5A and 5B, in a case of two step CF RACH, a gNB 503signals contention free random access resources (i.e., dedicatedpreambles/RACH occasions and PUSCH resources) to a UE 501 in steps 510and 550, respectively. The UE 501 transmits a PRACH preamble in a PRACHoccasion and an msgA payload (or an msgA MAC PDU) in a PUSCHresource/occasion using the assigned contention free random accessresources in steps 520, 530, 560, and 570, respectively. The UE 501stores the msgA payload (i.e., the msgA MAC PDU) in an msgA buffer. FormsgA payload transmission, a HARQ entity in the UE 501 perform a newHARQ packet transmission wherein HARQ packet for transmission isgenerated using the msgA payload in the msgA buffer. The UE 501maintains a number of parallel HARQ processes. Each HARQ processsupports one transport block (TB). Each HARQ process is associated witha HARQ process identifier. For UL transmission for the msgA payload, aHARQ process identifier 0 is used.

After transmitting the msgA, the UE 501 starts msgB-ResponseWindow andmonitors the PDCCH for random access response (i.e., an msgB) inmsgB-ResponseWindow. The UE 501 monitors a PDCCH of an SpCell for arandom access response identified by an MSGB-RNTI while themsgB-ResponseWindow is running. If a C-RNTI MAC CE was included in themsgA payload, the UE 501 additionally monitors the PDCCH of the SpCellfor a random access response identified by the C-RNTI while themsgB-ResponseWindow is running.

If the gNB 503 receives the msgA (i.e., both the PRACH preamble and themsgA payload) successfully, the gNB 503 includes the C-RNTI of the UE501 (which transmitted the msgA) in the msgB as shown in FIG. 5A in step540. The gNB 503 may identify the UE 501 from the received PRACHpreamble. The PRACH preamble is received in a contention free randomaccess resource assigned to the UE 501. However, if the gNB 503 onlyreceives the PRACH preamble but fails to receive the msgA payload, thegNB 503 does not include the C-RNTI in the msgB as shown in FIG. 5B instep 580. The msgB includes a RAPID corresponding to the PRACH preambleirrespective of whether the gNB 503 receives the msgA payload or not.

If the msgA is transmitted using the contention free random accessresource and if the msgB received by the UE 501 includes the RAPIDcorresponding to the PRACH preamble transmitted by the UE 501, then theUE 501 considers that the msgB is successfully received and the randomaccess procedure is successfully completed.

If the msgB includes a C-RNTI, the UE 501 discards the msgA payload,i.e., the UE 501 discards the msgA buffer and an HARQ buffer used formsgA payload transmission.

If the msgB does not include the C-RNTI, the UE 501 retransmits the msgApayload (i.e., an msgA MAC PDU) in the UL grant received in the msgB.For msgA payload transmission in the UL grant received in the msgB, anHARQ entity in the UE 501 performs a new HARQ packet transmissionwherein a HARQ packet for transmission is generated using the msgApayload in the msgA buffer. For UL transmission for the msgA payload, anHARQ process identifier 0 is used. Upon transmitting the msgA payload inthe UL grant received in the msgB, if the UE 501 receives a PDCCHaddressed to C-RNTI for an HARQ process zero with new data indicator(NDI) not toggled, the UE 501 performs HARQ retransmission. If the msgBdoes not include the C-RNTI and the UL grant is not received in themsgB, the UE 501 retransmits the msgA payload in a subsequent UL grantusing an HARQ process zero upon receiving a PDCCH addressed to a C-RNTIfor an HARQ process zero with an NDI not toggled wherein the UE 501performs an HARQ retransmission.

FIGS. 6A and 6B are flow-diagrams of a two-step random access procedureaccording to an embodiment.

Referring to FIGS. 6A and 6B, in a case of two step CF RACH, a gNB 603signals contention free random access resources (i.e., dedicatedpreambles/RACH occasions and PUSCH resources) to a UE 601 in steps 610and 650, respectively. The UE 601 transmits a PRACH preamble in a PRACHoccasion and an msgA payload (or an msgA MAC PDU) in a PUSCHresource/occasion using the assigned contention free random accessresources in steps 620, 630, 660, and 670, respectively. The UE 601stores the msgA payload (i.e., the msgA MAC PDU) in an msgA buffer. FormsgA payload transmission, an HARQ entity in the UE 601 performs a newHARQ packet transmission wherein an HARQ packet for transmission isgenerated using the msgA payload in an msgA buffer. The UE 601 maintainsa number of parallel HARQ processes. Each HARQ process supports one TB.Each HARQ process is associated with an HARQ process identifier. For ULtransmission for an msgA payload, an HARQ process identifier 0 is used.

After transmitting the msgA, the UE 601 starts an msgB-ResponseWindowand monitors the PDCCH for a random access response (i.e., the msgB) inthe msgB-ResponseWindow. The UE 601 monitors a PDCCH of an SpCell for arandom access response identified by an MSGB-RNTI while themsgB-ResponseWindow is running. If a C-RNTI MAC CE was included in themsgA payload, the UE 601 additionally monitors the PDCCH of the SpCellfor a random access response identified by the C-RNTI while themsgB-ResponseWindow is running.

If the gNB 603 receives the msgA (i.e., both a PRACH preamble and anmsgA payload) successfully, the gNB 603 does not include the UL grant inthe msgB as shown in FIG. 6A in step 640. If the gNB 603 only receivesthe PRACH preamble but fails to receive the msgA payload, the gNB 603includes the UL grant in the msgB as shown in FIG. 6B in step 680. ThemsgB includes a RAPID corresponding to the PRACH preamble irrespectiveof whether the gNB 603 receives the msgA payload or not.

If the msgA is transmitted using the contention free random accessresource and if the msgB received by the UE 601 includes a RAPIDcorresponding to the PRACH preamble transmitted by the UE 601 then theUE 601 considers that the msgB is successfully received and the randomaccess procedure is successfully completed.

If the msgB does not include a UL grant, the UE 601 discards the msgApayload, i.e., discards the msgA buffer and an HARQ buffer used for msgApayload transmission.

If the msgB includes a UL grant, the UE 601 retransmits the msgA payloadin the UL grant received in the msgB. For msgA payload transmission inthe UL grant received in the msgB, an HARQ entity in the UE 601 performsa new HARQ packet transmission wherein an HARQ packet for transmissionis generated using the msgA payload in the msgA buffer. For ULtransmission for an msgA payload, an HARQ process identifier 0 is used.Upon transmitting the msgA payload in the UL grant received in the msgB,if the UE 601 receives a PDCCH addressed to a C-RNTI for an HARQ processzero with an NDI not toggled, the UE 601 performs an HARQretransmission.

FIGS. 7A and 7B are flow-diagrams of a two-step random access procedureaccording to an embodiment.

Referring to FIGS. 7A and 7B, in a case of two step CF RACH, a gNB 703signals contention free random access resources (i.e., dedicatedpreambles/RACH occasions and PUSCH resources) to a UE 701 in steps 710and 750, respectively. The UE 701 transmits a PRACH preamble in a PRACHoccasion and a msgA payload (or a msgA MAC PDU) in a PUSCHresource/occasion using the assigned contention free random accessresources in steps 720, 730, 760, and 770, respectively. The UE 701stores the msgA payload (i.e., the msgA MAC PDU) in an msgA buffer. FormsgA payload transmission, an HARQ entity in the UE 701 performs a newHARQ packet transmission wherein an HARQ packet for transmission isgenerated using the msgA payload in the msgA buffer. The UE 701maintains a number of parallel HARQ processes. Each HARQ processsupports one TB. Each HARQ process is associated with an HARQ processidentifier. For UL transmission for the msgA payload, an HARQ processidentifier 0 is used.

After transmitting the msgA, the UE 701 starts an msgB-ResponseWindowand monitors the PDCCH for a random access response (i.e., an msgB) inan msgB-ResponseWindow. The UE 701 monitors a PDCCH of an SpCell for arandom access response identified by MSGB-RNTI while themsgB-ResponseWindow is running. If a C-RNTI MAC CE was included in themsgA payload, the UE 701 additionally monitors the PDCCH of the SpCellfor a random access response identified by the C-RNTI while themsgB-ResponseWindow is running.

If the gNB 703 receives the msgA (i.e., both the PRACH preamble and themsgA payload) successfully, the gNB 703 sets a NEW_TX bit to 1 in themsgB as shown in FIG. 7A in step 740). If the gNB 701 only receives thePRACH preamble but fails to receive the msgA payload, the gNB 703 setsthe NEW_TX bit to 0 in the msgB as shown in FIG. 7B in step 780. ThemsgB includes a RAPID corresponding to the PRACH preamble irrespectiveof whether the gNB 703 receives the msgA payload or not.

If the msgA is transmitted using the contention free random accessresources and if the msgB received by the UE 701 includes the RAPIDcorresponding to the PRACH preamble transmitted by the UE 701 then theUE 701 considers that the msgB is successfully received and the randomaccess procedure is successfully completed.

If the msgB includes the NEW_TX bit set to 1 in the msgB, the UE 701discards the msgA payload, i.e., discards the msgA buffer and an HARQbuffer used for msgA payload transmission. The UE 701 transmits a newMAC PDU in a UL grant received in the msgB.

If the msgB includes the NEW_TX bit set to 0, the UE 701 retransmits themsgA payload in the UL grant received in the msgB. For msgA payloadtransmission in the UL grant received in the msgB, an HARQ entity in theUE 701 performs a new HARQ packet transmission wherein an HARQ packetfor transmission is generated using the msgA payload in the msgA buffer.For UL transmission for the msgA payload, an HARQ process identifier 0is used. Upon transmitting the msgA payload in the UL grant received inthe msgB, if the UE 701 receives a PDCCH addressed to a C-RNTI for anHARQ process zero with an NDI not toggled, the UE 701 performs an HARQretransmission. If the msgB includes the NEW_TX bit set to 0 and a ULgrant is not received in the msgB, the UE 701 retransmits the msgApayload in a subsequent UL grant using an HARQ process zero uponreceiving a PDCCH addressed to a C-RNTI for an HARQ process zero with anNDI not toggled.

FIGS. 8A and 8B are flow-diagrams of a two-step random access procedureaccording to an embodiment.

Referring to FIGS. 8A and 8B, in a case of two-step CF RACH, a gNB 803signals contention free random access resources (i.e., dedicatedpreambles/RACH occasions and PUSCH resources) to a UE 801 in steps 810and 850, respectively. The UE 801 transmits a PRACH preamble in a PRACHoccasion and an msgA payload (or an msgA MAC PDU) in a PUSCHresource/occasion using the assigned contention free random accessresources in steps 820, 830, 860, and 870, respectively. The UE 801stores the msgA payload (i.e., an msgA MAC PDU) in an msgA buffer. FormsgA payload transmission, an HARQ entity in the UE 801 perform a newHARQ packet transmission wherein an HARQ packet for transmission isgenerated using the msgA payload in the msgA buffer. The UE 801maintains a number of parallel HARQ processes. Each HARQ processsupports one TB. Each HARQ process is associated with an HARQ processidentifier. For UL transmission for the msgA payload, an HARQ processidentifier 0 is used.

After transmitting the msgA, the UE 801 starts an msgB-ResponseWindowand monitors the PDCCH for a random access response (i.e., an msgB) inan msgB-ResponseWindow. The UE 801 monitors a PDCCH of an SpCell for arandom access response identified by an MSGB-RNTI while themsgB-ResponseWindow is running. If a C-RNTI MAC CE was included in themsgA payload, the UE 801 additionally monitors the PDCCH of the SpCellfor a random access response identified by the C-RNTI while themsgB-ResponseWindow is running.

If the gNB 803 receives the msgA (i.e., both the PRACH preamble and themsgA payload) successfully, the gNB 803 includes a RAPID, a TA command,a UL grant, a C-RNTI of the UE 801 (which transmitted the msgA) in themsgB as illustrated in FIG. 8A in step 840. A PDCCH for the msgB isaddressed to MSGB-RNTI. Upon receiving the msgB wherein the RAPIDcorresponds to the random access preamble transmitted by the UE 801, therandom access procedure is successfully completed. In this case, the UE801 discards the msgA payload, i.e., discards the msgA buffer and anHARQ buffer used for the msgA payload transmission.

The gNB 803 may identify the UE 801 from the received PRACH preamble.The PRACH preamble is received in a contention free resource assigned tothe UE 801. If the gNB 803 only receives the PRACH preamble but fails toreceive the msgA payload, the gNB 80-3 includes the RAPID, the TAcommand, the UL grant and a temporary C-RNTI (a TC-RNTI) in the msgB asshown in FIG. 8B in step 880.

If the msgB includes the RAPID, the TA command, the UL grant and theTC-RNTI wherein the RAPID corresponds to the random access preambletransmitted by the UE 801, the random access procedure is successfullycompleted. The UE 801 retransmits the msgA payload in the UL grantreceived in the msgB. For msgA payload transmission in the UL grantreceived in the msgB, an HARQ entity in the UE 801 performs a new HARQpacket transmission wherein an HARQ packet for transmission is generatedusing the msgA payload in the msgA buffer. For UL transmission for themsgA payload, an HARQ process identifier 0 is used. Upon transmittingthe msgA payload in the UL grant received in the msgB, if the UE 801receives a PDCCH addressed to a C-RNTI for an HARQ process zero with anNDI not toggled, the UE 801 performs an HARQ retransmission. If a ULgrant is not received in the msgB, the UE 801 retransmits the msgApayload in a subsequent UL grant using an HARQ process zero uponreceiving a PDCCH addressed to a C-RNTI for an HARQ process zero with anNDI not toggled.

If the gNB 803 receives the msgA (i.e., both the PRACH preamble and themsgA payload) successfully, the gNB 803 transmits a PDCCH addressed to aC-RNTI of the UE 801 (which transmitted the msgA), wherein the PDCCH mayschedule a UL grant for a new transmission or the PDCCH may include a DLassignment. Upon receiving the PDCCH, the random access procedure issuccessfully completed. In this case, the UE 801 discards the msgApayload, i.e., discards the msgA buffer and an HARQ buffer used for msgApayload transmission.

The gNB 803 may identify the UE 801 from the received PRACH preamble.The PRACH preamble is received in a contention free resource assigned tothe UE 801. If the gNB 803 only receives the PRACH preamble but fails toreceive the msgA payload, the gNB 803 includes a RAPID, a TA command, aUL grant and a TC-RNTI in an msgB. The PDCCH for the msgB is addressedto an MSGB-RNTI.

If the msgB includes the RAPID, the TA command, the UL grant and theTC-RNTI wherein the RAPID corresponds to random access preambletransmitted by the UE 801, the random access procedure is successfullycompleted. The UE 801 retransmits the msgA payload in the UL grantreceived in the msgB. For msgA payload transmission in the UL grantreceived in the msgB, an HARQ entity in the UE 801 performs a new HARQpacket transmission wherein an HARQ packet for transmission is generatedusing the msgA payload in the msgA buffer. For a UL transmission for themsgA payload, an HARQ process identifier 0 is used. Upon transmittingthe msgA payload in the UL grant received in the msgB, if the UE 801receives a PDCCH addressed to a C-RNTI for an HARQ process zero with anNDI not toggled, the UE 801 performs an HARQ retransmission.

If the gNB 803 receives the msgA (i.e., both the PRACH preamble and themsgA payload) successfully, the gNB 803 transmits a PDCCH addressed to aC-RNTI of the UE 801 (which transmitted the msgA), wherein the PDCCH mayschedule a UL grant for a new transmission or a DL assignment. Uponreceiving the PDCCH, the random access procedure is successfullycompleted. In this case, the UE 801 discards the msgA payload, i.e.,discards the msgA buffer. However, the HARQ buffer used for msgA payloadtransmission is not discarded.

The gNB 803 may identify the UE 801 from the received PRACH preamble.The PRACH preamble is received in a contention free resource assigned tothe UE 801. If the gNB 803 only receives the PRACH preamble but fails toreceive the msgA payload, the gNB 803 transmits a PDCCH addressed to aC-RNTI of the UE 801, wherein the PDCCH may schedule a UL grant for anew transmission or a DL assignment. Upon receiving the PDCCH, therandom access procedure is successfully completed. In this case, the UE801 discards the msgA payload, i.e., discards the msgA buffer. However,the HARQ buffer used for msgA payload transmission is not discarded.

Upon completion of the random access procedure, if the gNB 803 has notreceived the msgA payload during the random access procedure, the gNB803 will transmit a PDCCH addressed to a C-RNTI for an HARQ process zerowith an NDI not toggled. If the UE 801 receives the PDCCH addressed tothe C-RNTI for the HARQ process zero with the NDI not toggled, the UE801 performs an HARQ retransmission.

Upon completion of random access procedure:

-   -   if this random access procedure was completed upon receiving        fallbackRAR (fallback RAR is received in msgB and consists of        RAPID, TA, TC-RNTI and UL grant):        -   UE does not flush the HARQ buffer used for transmission of            the MAC PDU in the Msg3 buffer and the msgA buffer;    -   if this random access procedure was not completed upon receiving        fallbackRAR        -   flush the HARQ buffer used for transmission of the MAC PDU            in the msg3 buffer and the msgA buffer

In an embodiment of the proposed invention, upon completion of randomaccess procedure:

-   -   if last random access preamble transmitted was not selected from        contention based preambles:        -   UE does not flush the HARQ buffer used for transmission of            the MAC PDU in the Msg3 buffer and the msgA buffer;    -   Else:        -   flush the HARQ buffer used for transmission of the MAC PDU            in the msg3 buffer and the msgA buffer

In an embodiment of the proposed invention, upon completion of randomaccess procedure:

-   -   if last random access preamble transmitted was not selected from        contention based preambles and:        -   UE does not flush the HARQ buffer used for transmission of            the MAC PDU in the Msg3 buffer and the msgA buffer;    -   Else:        -   flush the HARQ buffer used for transmission of the MAC PDU            in the msg3 buffer and the msgA buffer

FIGS. 9A and 9B are flow-diagrams of a two-step random access procedureaccording to an embodiment.

Referring to FIGS. 9A and 9B, in a case of two step CF RACH, an gNB 903signals contention free random access resources (i.e., dedicatedpreambles/RACH occasions and PUSCH resources) to a UE 901 in steps 910and 960, respectively. The UE 901 transmits a PRACH preamble in a PRACHoccasion and an msgA payload (or an msgA MAC PDU) in a PUSCHresource/occasion using the assigned contention free random accessresources in steps 920, 930, 970, and 980, respectively. The UE 901stores the msgA payload (i.e., an msgA MAC PDU) in an msgA buffer. FormsgA payload transmission, an HARQ entity in the UE 901 perform a newHARQ packet transmission wherein an HARQ packet for transmission isgenerated using the msgA payload in the msgA buffer. The UE 901maintains a number of parallel HARQ processes. Each HARQ processsupports one TB. Each HARQ process is associated with an HARQ processidentifier. For UL transmission for the msgA payload, an HARQ processidentifier 0 is used.

After transmitting the msgA, the UE 901 starts an msgB-ResponseWindowand monitors the PDCCH for a random access response (i.e., an msgB) inthe msgB-ResponseWindow. The UE 901 monitors a PDCCH of an SpCell for arandom access response identified by an MSGB-RNTI while themsgB-ResponseWindow is running. If a C-RNTI MAC CE was included in themsgA payload, the UE 901 additionally monitors the PDCCH of the SpCellfor a random access response identified by the C-RNTI while themsgB-ResponseWindow is running.

If the gNB 901 receives the msgA (i.e., both the PRACH preamble and themsgA payload) successfully, the gNB 901 transmits a RAPID and a TAcommand in the msgB as shown in FIG. 9A in step 940. Subsequently, thegNB 903 transmits a PDCCH addressed to a C-RNTI of the UE 901 schedulingan UL grant and an NDI bit in a DCI is toggled for an HARQ process ID(e.g. an HARQ process zero) reserved for the msgA in step 950. If thegNB 903 only receives the PRACH preamble but fails to receive the msgApayload, the gNB 903 transmits the RAPID and a TA command in the msgB asshown in FIG. 9B in step 990. Subsequently, the gNB 903 transmits aPDCCH addressed to a C-RNTI of the UE 901 scheduling an UL grant and anNDI bit in DCI is not toggled for an HARQ process ID reserved for themsgA in step 995. The gNB 903 may identify the UE 901 from the receivedPRACH preamble. The PRACH preamble is received in a contention freeresource assigned to a specific UE.

If the msgA is transmitted using the contention free resource and if themsgB received by the UE 901 includes a RAPID corresponding to the PRACHpreamble transmitted by the UE 901, then the UE 901 considers that themsgB is successfully received and a random access procedure issuccessfully completed.

Upon successful completion of a random access procedure, if the UE 901receives a PDCCH addressed to a C-RNTI scheduling an UL grant and an NDIbit in a DCI is toggled for an HARQ process ID reserved for an msgA, theUE 901 discards the msgA payload. The UE 901 transmits a new MAC PDU inthe received UL grant.

Upon successful completion of a random access procedure, if the UE 901receives a PDCCH addressed to a C-RNTI scheduling an UL grant and an NDIbit in a DCI is not toggled for an HARQ process ID reserved for an msgA,the UE 901 retransmits the msgA payload in the received UL grant.

FIGS. 10A and 10B are flow-diagrams of a two-step random accessprocedure according to an embodiment.

Referring to FIGS. 10A and 10B, in a case of two step CF RACH, a gNB1003 signals contention free random access resources (i.e., dedicatedpreambles/RACH occasions and PUSCH resources) to a UE 1001 in steps 1010and 1050, respectively, the UE 1001 transmits an msgA PRACH preamble andan msgA payload in steps 1020, 1030, 1060, and 1070, respectively. TheUE 1001 stores the msgA payload (i.e., a MAC PDU) in an msgA buffer. FormsgA payload transmission, an HARQ entity in the UE 1001 performs a newHARQ packet transmission wherein an HARQ packet for transmission isgenerated using the msgA payload in the msgA buffer. The UE 1001maintains a number of parallel HARQ processes. Each HARQ processsupports one TB. Each HARQ process is associated with an HARQ processidentifier. For UL transmission for the msgA payload, an HARQ processidentifier 0 is used.

After transmitting the msgA, the UE 1001 starts an msgB-ResponseWindowand monitors the PDCCH for a random access response (i.e., an msgB) inan msgB-ResponseWindow. The UE 1001 monitors a PDCCH of an SpCell for arandom access response identified by an MSGB-RNTI while themsgB-ResponseWindow is running. If a C-RNTI MAC CE was included in themsgA payload, the UE 1003 additionally monitors the PDCCH of the SpCellfor a random access response identified by the C-RNTI while themsgB-ResponseWindow is running.

If the gNB 1003 receives the msgA (i.e., both the PRACH preamble and themsgA payload) successfully, the gNB 1003 transmits a RAPID, a TA acommand and a UL grant in the msgB as shown in FIG. 10A in step 1040.The HARQ process ID other than zero is included in the msgB. If the gNB1003 only receives the PRACH preamble but fails to receive the msgApayload, the gNB 1003 transmits the RAPID, the TA command and the ULgrant in msgB as shown in FIG. 10B in step 1080. The HARQ process IDzero is included in the msgB in this case.

If the msgA is transmitted using the contention free resource and if themsgB received by the UE 1001 includes a RAPID corresponding to the PRACHpreamble transmitted by the UE 1001 then the UE 1001 considers that themsgB is successfully received and the random access procedure issuccessfully completed.

If the msgB includes an HARQ process ID other than zero, the UE 1001discards the msgA payload. The UE 1001 transmits a new MAC PDU in the ULgrant received in the msgB.

If the msgB includes an HARQ process ID zero, the UE 1001 retransmitsthe msgA payload in the UL grant received in the msgB. For msgA payloadtransmission in the UL grant received in the msgB, an HARQ entity in theUE 1001 performs a new HARQ packet transmission wherein an HARQ packetfor transmission is generated using the msgA payload in the msgA buffer.For UL transmission for the msgA payload, an HARQ process identifier 0is used. Upon transmitting the msgA payload in the UL grant received inthe msgB, if the UE 1001 receives a PDCCH addressed to a C-RNTI for anHARQ process zero with an NDI not toggled, the UE 1001 performs an HARQretransmission. If the msgB includes an HARQ process ID zero and if anUL grant is not received in the msgB, the UE 1001 retransmits the msgApayload in a subsequent UL grant using an HARQ process zero uponreceiving a PDCCH addressed to a C-RNTI for an HARQ process zero with anNDI not toggled.

2 Step RACH: Msg3 MAC PDU Generation During Fallback

FIG. 11 is a flow-diagram of a contention based two-step random accessprocedure according to an embodiment.

Referring to FIG. 11, in a case of two-step contention based RACH, if agNB 1103 only receives a PRACH preamble but fails to receive an msgApayload in steps 1110 and 1120, the gNB 1103 may indicate to the UE 1101to fallback to a four step RACH. For indicating the fallback, the gNB1103 transmits a RAPID, a TA command, a UL grant and a TC-RNTI in anmsgB as shown in FIG. 11 step 1130. Upon receiving the fallbackindication in the msgB, the UE 1101 generates and transmits an msg3 in aUL grant received in an msgB in step 1140. The contention resolution isperformed via an msg4 as in a four step RACH procedure in step 1150.

In a four step RACH upon receiving a UL grant in an msg2 a new MAC PDUis generated for an msg3 using an available MAC CE(s) and an SDU(s) in alogical channel buffer(s) in a MAC. In a case of two step RACH, therequired information is not available in a logical channel buffer(s) asit is already transmitted in the msgA.

FIG. 12 is a flowchart of a contention based two-step random accessprocedure according to an embodiment.

Referring to FIG. 12, a UE stores a MAC PDU transmitted in an msgA. Ifan RA procedure is successfully completed upon receiving an msgB, the UEdiscards the stored msgA MAC PDU. If the msgB indicates fall back, theUE prepares an msg3 MAC PDU using contents of an msgA MAC PDU.

The UE transmits a PRACH preamble in a PRACH occasion and an msgApayload (or an msgA MAC PDU) in a PUSCH resource/occasion in step 1210.A PRACH preamble is a contention based preamble. The UE stores the msgApayload (i.e., the msgA MAC PDU) in an msgA buffer in step 1220. FormsgA payload transmission, an HARQ entity in the UE performs a new HARQpacket transmission wherein an HARQ packet for transmission is generatedusing the msgA payload in the msgA buffer. The UE maintains a number ofparallel HARQ processes. Each HARQ process supports one TB. Each HARQprocess is associated with an HARQ process identifier. For ULtransmission for the msgA payload, an HARQ process identifier 0 is used.

After transmitting the msgA, the UE starts a msgB-ResponseWindow andmonitors a PDCCH for a random access response (i.e., the msgB) in anmsgB-ResponseWindow. The UE monitors a PDCCH of an SpCell for a randomaccess response identified by an MSGB-RNTI while the msgB-ResponseWindowis running. If a C-RNTI MAC CE was included in the msgA payload, the UEadditionally monitors the PDCCH of the SpCell for a random accessresponse identified by the C-RNTI while the msgB-ResponseWindow isrunning.

If the UE receives the msgB in step 1230, and the msgB includes fallbackinformation, i.e., a RAPID, a TA, a TC-RNTI and a UL grant and the RAPIDin the fallback information matches the random access preamble index ofthe PRACH preamble transmitted by the UE in step 1240, the UE preparesan msg3 MAC PDU using contents of the msgA MAC PDU and transmits themsg3 MAC PDU in the UL grant received in the msgB in step 1260.Otherwise, if the msgB does not include fallback information and the RAprocedure is successfully completed upon receiving the msgB, the UEdiscards the msgA payload in steps 1240 and 1250.

For msg3 payload transmission in the UL grant received in the msgB, anHARQ entity in the UE performs a new HARQ packet transmission wherein anHARQ packet for transmission is generated using the msg3 payload. For ULtransmission for the msg3 payload, an HARQ process identifier 0 is used.Upon transmitting the msg3 payload in the UL grant received in the msgB,if the UE receives a PDCCH addressed to a TC-RNTI for an HARQ processzero, the UE performs an HARQ retransmission.

FIG. 13 is a flowchart of a contention based two-step random accessprocedure according to an embodiment.

Referring to FIG. 13, when a received msgB indicates fallback in step1310, a UE generates an msg3 MAC PDU using contents of an msgA MAC PDUas follows.

If a UL grant size in the msgB is the same as the size of the msgA MACPDU, the UE uses the msgA MAC PDU as the msg3 MAC PDU in steps 1320 and1360.

If the UL grant size in the msgB is greater than the size of the msgAMAC PDU in step 1330, the UE generates a new MAC PDU by adding paddingat the end of the msgA MAC PDU in step 1340. Alternatively, the UEgenerates a new MAC PDU using contents of the msgA MAC PDU and availableMAC CE(s) and SDU(s) in a logical channel buffer(s) by applying alogical channel prioritization (LCP) in step 1340.

Alternatively, the UE generates a new MAC PDU using a MAC subPDU(s)carrying a MAC SDU from the msgA MAC PDU and available MAC CE(s) andSDU(s) in a logical channel buffer(s) by applying an LCP. In otherwords, the UE indicates to the multiplexing and assembly entity toinclude a MAC subPDU(s) carrying a MAC SDU from the obtained msgA MACPDU in a subsequent uplink transmission in step 1340.

Alternatively, a MAC of the UE may inform the RLC entities of a logicalchannel(s) whose MAC SDU(s) is included in the msgA MAC PDU that thoseMAC SDUs are lost. The lost MAC SDU(s) may also be provided to RLCentities by a MAC. RLC entities may then retransmit those MAC SDU(s) instep 1340.

If a UL grant size in the msgB is less than the size of a MsgA MAC PDUin step 1330, the UE generates a new MAC PDU using contents of the msgAMAC PDU and an available MAC CE(s) and an SDU(s) in a logical channelbuffer(s) by applying LCP. The UE discards the msgA MAC PDU in step1350.

Alternatively, the UE generates a new MAC PDU using a MAC subPDU(s)carrying a MAC SDU from the msgA MAC PDU and an available MAC CE(s) andan SDU(s) in a logical channel buffer(s) by applying LCP. In otherwords, the UE indicates to the multiplexing and assembly entity toinclude a MAC subPDU(s) carrying a MAC SDU from the obtained msgA MACPDU in a subsequent uplink transmission in step 1350.

Alternatively, a MAC of the UE may inform the RLC entities of a logicalchannel(s) whose MAC SDU(s) is included in the msgA MAC PDU that thoseMAC SDUs are lost. The lost MAC SDU(s) may also be provided to RLCentities by the MAC. RLC entities may then retransmit those MAC SDU(s)in step 1350.

MsgB Reception Timing

FIG. 14 is an illustration of a message A (or msgA) reception timingaccording to an embodiment.

Referring to FIG. 14, in a two-step RACH, a channel structure of themsgA includes a PRACH preamble and a PUSCH carrying a payload 1410 and1420. The PRACH preamble and the PUSCH (or, additionally a GAP 1425 inthe msgA are time division multiplexed (TDMed). Thus, instead ofstarting an msgB-ResponseWindow from the end of the random accesspreamble transmission in msgA, an msgB-ResponseWindow for an msgBreception starts at the first PDCCH occasion for the msgB reception thatis at least one symbol away from the end of a PUSCH transmission in themsgA. In other words, the msgB-ResponseWindow for msgB reception startsat the first PDCCH occasion for msgB reception that is at least onesymbol after the last symbol of the PUSCH occasion corresponding to thePUSCH transmission. The PDCCH occasions for msgB reception areconfigured by ra-SearchSpace parameters in PDCCH-ConfigCommoninformation element (IE) of the BWP used for RA procedure.

During the two step random access procedure, if only a PRACH preamble istransmitted by the UE during the msgA transmission, anmsgB-ResponseWindow starts at the first PDCCH occasion for a randomaccess response (i.e., the msgB) that is at least one symbol away fromthe end of the random access preamble transmission. The msgA payload ina PUSCH occasion is not transmitted if there is no available PUSCHoccasion corresponding to a PRACH preamble and a PRACH occasion selectedby UE. An msgA payload is not transmitted if a channel is not availableduring a PUSCH occasion in case a UL transmission is on an unlicensedspectrum. If both a PRACH preamble and a PUSCH are transmitted by theUE, an msgB-ResponseWindow for a random access response (i.e., themsgB), reception starts at the first PDCCH occasion for a responsereception that is at least one symbol away from the end of a PUSCH inthe msgA.

PUSCH Resource Signaling for MsgA Payload in 2 Step RACH

FIG. 15 is an illustration of PUSCH resource signaling according to anembodiment.

Referring to FIG. 15, a gNB signals parameters N2, N3, L, O andMsgAPUSCH-FrequencyStart. These parameters are signaled for each BWPsupporting a two-step RACH. These parameters may be signaled as part ofa two-step RACH configuration in system information or dedicated RRCsignaling. Using these parameters, a UE may identify PUSCH resourceswith respect to each set of frequency division multiplexed (FDMed) PRACHoccasions as shown in FIG. 15. The set of FDMed PRACH occasions refersto RACH occasions of a PRACH slot. The slots having PRACH occasions areconfigured by parameter PRACH configuration (config) index as in thelegacy four step RACH. MsgAPUSCH-FrequencyStart indicates a starting PRBof FDMed PUSCH resources in the frequency domain. The starting PRB isindicated with respect to PRB 0 of BWP. N2 is a number of FDMed PUSCHresources. N3 is a number of PRBs in one PUSCH resource. L is a lengthin symbols and/or slots of a PUSCH resource. O is an offset of a PUSCHresource with respect to a PRACH occasion, i.e., with respect to a startor an end of a PRACH slot carrying the PRACH occasion. The offset may bein a unit of symbols or slots. The number of PUSCH resources mapped toone PRACH occasion equals N2/N1; for N2>=N1 where N1 is a number ofFDMed PRACH occasions. For N2 less than N1, N1/N2 PRACH occasions aremapped to one PUSCH resource. In this case, preambles used in PRACHoccasions mapped to the same PUSCH resource should be different.

FIG. 16 is an illustration of PUSCH resource signaling according to anembodiment.

Referring to FIG. 16, a gNB signals parameters N2, N3, L, O andMsgAPUSCH-FrequencyStart. These parameters may be signaled as part of atwo-step RACH configuration in system information or dedicated RRCsignaling. These parameters are signaled for each BWP supporting atwo-step RACH. Using these parameters, a UE may identify PUSCH resourceswith respect to each set of FDMed/time division multiplexed (TDMed)PRACH occasions as shown in FIG. 16. The set of PRACH occasions refersto RACH occasions of a PRACH slot. MsgAPUSCH-FrequencyStart indicates astarting PRB of FDMed PUSCH resources in the frequency domain. Thestarting PRB is indicated with respect to PRB 0 of BWP. N2 is a numberof FDMed PUSCH resources. N3 is a number of PRBs in one PUSCH resource.L is a length in symbols and/or slots of a PUSCH resource. O is anoffset of PUSCH resource with respect to a PRACH occasion, i.e., withrespect to a start or an end of a PRACH slot carrying the PRACHoccasion. The offset may be in a unit of symbols or slots. The number ofPUSCH resources mapped to one PRACH occasion equals N2/M; for N2>=M; Mequals N1*N4 where N1 is a number of FDMed PRACH occasions and N4 is anumber of PRACH occasions in one PRACH slot. For N2 less than M, M/N2PRACH occasions are mapped to one PUSCH resource. In this case,preambles used in PRACH occasions mapped to the same PUSCH resourceshould be different.

RACH Less Handover or Reconfiguration

In NR TCI, a framework is specified for a beam indication/switch for aPDCCH, a PDSCH and a channel state information reference signal(CSI-RS). One DL RS (SSB or CSI-RS) is RRC configured per TCI state. AUE identifies its beam information based on the DL RS in the indicatedTCI state. For PDCCH beam indication, a set of TCI states are RRCconfigured per CORESET. One TCI state amongst these based on which UEdetermines the PDCCH beam is indicated by a MAC CE.

In a case of handover, the UE is configured with a list of TCI statesfor a target cell. The configured TCI states are initially deactivatedupon handover. If the UE received an initial configuration of more thanone TCI states for PDCCH receptions by a higher layer parameterTCI-States but has not received a MAC CE activation command for one ofthe TCI states, the UE assumes that the DM-RS antenna port associatedwith PDCCH receptions is quasi co-located with the SS/PBCH block the UEidentified during the random access procedure at the target cell.

A RACH-less handover is being discussed to reduce handover latency. In acase of a RACH-less handover, since a UE does not perform random accessat the target cell, the UE knows how to receive a PDCCH in the targetcell as all TCI states received in a handover command are in adeactivated state.

If a RACH-less handover is indicated in a reconfiguration message, angNB indicates a single TCI state for a target cell. In a case of a BWPoperation, in a DL BWP indicated by a first active DL BWP ID, a gNBconfigures only a single TCI state if the RACH-less handover isindicated in a reconfiguration message. The UE uses the TCI stateinformation provided by the TCI state to receive a PDCCH in the targetcell upon handover. The UE assumes that the DM-RS antenna portassociated with PDCCH receptions is quasi co-located with the one ormore DL RS configured by the TCI state.

The gNB may configure the single TCI state based on an SSB ID/CSI RS IDof the target cell included in a last measurement report. As a result,the single TCI state provided in a reconfiguration message may not bevalid when the UE performs a handover to the target cell. The followingmay be considered upon handover:

1) If a RACH-less handover is configured and if an SSB ID/CSI RS IDindicated by a TCI state is not suitable, the UE performs random access.

2) If the UE fails to receive any PDCCH for a configurable duration, theUE performs random access.

If a RACH-less handover is indicated in a reconfiguration message, a gNBindicates an activated TCI state (i.e., the gNB signals an activated TCIstate ID in a reconfiguration message) amongst a list of TCI statesconfigured for a target cell. In a case of a BWP operation, in a DL BWPindicated by a first active DL BWP ID, the gNB indicates an activatedTCI state (i.e., the gNB signals an activated TCI state ID in areconfiguration message) amongst the list of TCI states configured forthe target cell if a RACH-less handover is indicated in thereconfiguration message. A pre-defined (e.g. a first one) TCI state in alist of TCI states is active by default if a RACH-less handover isindicated in a reconfiguration message. The UE uses the TCI stateinformation provided by the activated TCI state to receive a PDCCH inthe target cell upon handover. The UE assumes that the DM-RS antennaport associated with PDCCH receptions is quasi co-located with the oneor more DL RS configured by the TCI state.

The gNB may indicate an activated TCI state based on an SSB ID/CSI RS IDof a target cell included in a last measurement report. As a result, theactivated TCI state provided in a reconfiguration message may not bevalid when the UE performs handover to the target cell. The followingmay be considered upon handover:

1) If a RACH-less handover is configured and if an SSB ID/CSI RS IDindicated by an activated TCI state is not suitable, a UE performs arandom access.

2) If the UE fails to receive any PDCCH based on an activated TCI statefor a configurable duration, the UE performs a random access.

If a RACH-less handover is indicated in a reconfiguration message and agNB indicates a list of TCI states for a target cell, a UE assumes thata DM-RS antenna port associated with PDCCH receptions is quasico-located with an SS/PBCH block or a CSI RS with a highest signalquality which the UE reported for the target cell in a last measurementreport.

1) If an SS/PBCH block or an CSI RS with a highest signal quality whichthe UE reported for the target cell in the last measurement report isnot suitable at the time of handover, the UE performs a random accesseven if a RACH-less handover is indicated in a reconfiguration message.

2) If the UE fails to receive any PDCCH for a configurable duration in atarget cell, the UE performs a random access.

Hereinafter, another embodiment of the disclosure, which relates to amethod and an apparatus of fallback to random access during a RACH-lesshandover is described.

In a 5G wireless communication system (also referred as NR), networkcontrolled cell level mobility is supported for UEs in RRC CONNECTED. Atypical procedure for cell level mobility is as follows:

1. A source gNB configures a UE measurement procedure and the UE reportsaccording to the measurement configuration.

2. The source gNB decides to handover the UE, based on aMeasurementReport and radio resource management (RRM) information.

3. The source gNB issues a Handover Request message to the target gNB,passing a transparent RRC container with necessary information toprepare the handover at the target side. The information includes atleast the target cell ID, KgNB*, the C-RNTI of the UE in the source gNB,RRM-configuration including UE inactive time, basic AS-configurationincluding antenna Info and DL Carrier Frequency, the current QoS flow toDRB mapping rules applied to the UE, the SIB1 from source gNB, the UEcapabilities for different radio access technologies (RATs), packet dataunit (PDU) session related information, and may include the UE reportedmeasurement information including beam-related information if available.The PDU session related information includes the slice information (ifsupported) and QoS flow level QoS profile(s).

4. Admission Control may be performed by the target gNB. Slice-awareadmission control shall be performed if the slice information is sent tothe target gNB. If the PDU sessions are associated with non-supportedslices the target gNB shall reject such PDU Sessions.

5. The target gNB prepares the handover with L1/L2 and sends theHANDOVER REQUEST ACKNOWLEDGE to the source gNB, which includes atransparent container to be sent to the UE as an RRC message to performthe handover.

6. The source gNB triggers the UE handover by sending anRRCReconfiguration message to the UE, containing information required toaccess the target cell: at least the target cell ID, the new C-RNTI, thetarget gNB security algorithm identifiers for the selected securityalgorithms. The RRCReconfiguration message may also include a set ofdedicated RACH resources, the association between RACH resources andSSB(s), the association between RACH resources and UE-specific CSI-RSconfiguration(s), common RACH resources, and system information of thetarget cell, etc.

7. The source gNB sends the SN STATUS TRANSFER message to the targetgNB.

8. The UE synchronizes to the target cell After the DL synchronization,UE performs random access procedure for UL synchronization. UE completesthe RRC handover procedure by sending RRCReconfigurationComplete messageto target gNB.

9. The target gNB sends a PATH SWITCH REQUEST message to accessmanagement function (AMF) to trigger 5G core (5GC) to switch the DL datapath towards the target gNB and to establish an NG-C interface instancetowards the target gNB.

10. 5GC switches the DL data path towards the target gNB. The UPF sendsone or more “end marker” packets on the old path to the source gNB perPDU session/tunnel and then can release any U-plane/tunnel (TNL)resources towards the source gNB.

11. The AMF confirms the PATH SWITCH REQUEST message with the PATHSWITCH REQUEST ACKNOWLEDGE message.

12. Upon reception of the PATH SWITCH REQUEST ACKNOWLEDGE message fromthe AMF, the target gNB sends the UE CONTEXT RELEASE to inform thesource gNB about the success of the handover. The source gNB may thenrelease radio and C-plane related resources associated to the UEcontext. Any ongoing data forwarding may continue.

In order to reduce handover latency, RACH-less handover is beingstudied. In a case of RACH-less handover, the UE is not required toperform random access with the target cell. RACH-less handover wassupported in the 4G wireless communication system. In the 4G wirelesscommunication system, a handover command (i.e., RRCReconfiguration)indicates whether the UE should skip a random access with a target cellor not. For UL transmission in a target cell, a handover command alsoindicates whether to apply timing advanced of source cell or TA equalszero. A handover command may optionally provide pre-allocated UL grantsfor transmitting the RRCReconfigurationComplete and/or UL data. The sameprocedure may be applied to RACH-less handover in NR. NR supports highfrequency bands (>6 GHz), also referred as FR2 frequency bands and thelower frequency bands (<=6 GHz) also referred as FR1 frequency bands. Athigh frequency, beamforming is essential. In the current handoverprocedure, initial beam alignment between a UE and a target cell occursvia a random access procedure.

In a case of RACH-less handover, for beam alignment, pre-allocated ULgrants signaled in a handover command may be associated with one or moreSSB/CSI RS(s). A UE may select an SSB/CSI RS and then transmit in a ULgrant corresponding to the selected SSB/CSI RS. A method is needed toselect an SSB/CSI RS amongst the SSB/CSI RSs associated withpre-allocated UL grants. A method is needed to handle the scenario whennone of the SSB/CSI RSs amongst the SSB/CSI RSs associated withpre-allocated UL grants may be selected.

Fallback Based on Measurement of SS-RSRP/CSI-RSRP of SSB/CSI-RS(s)Associated with the Pre-Allocated UL Grants

The various methods in the disclosure to handle the scenario when noneof the SSB/CSI RSs amongst the SSB/CSI RSs associated with pre-allocatedUL grants may be selected, are described below.

FIG. 17 is a flowchart of a method of handling a fallback to randomaccess during a random access channel less (RACH-less) handoveraccording to an embodiment.

Referring to FIG. 17, a UE 1701 receives an RRCReconfiguration messagefrom a source cell 1703 (e.g., a gNB) in step 1710. spCellConfig in theRRCReconfiguration message includes reconfigurationWithSync. Information(e.g. an indication to skip RACH) in reconfigurationWithSync IEindicates that the UE 1701 shall skip RACH towards a target cell 1705.The RRCReconfiguration message includes a pre-allocated UL grantconfiguration (parameters indicate periodically occurring UL grants).This configuration is provided for at least a UL BWP indicated by afirstActiveUplinkBWP-Id. The RRCReconfiguration message also includesinformation (i.e., SSB ID/CSI-RS ID) about an SSB/CSI-RS(s) associatedwith the pre-allocated UL grants.

The UE 1701 measures the SS-RSRP/CSI-RSRP of SSB/CSI-RS(s) associatedwith the pre-allocated UL grants in step 1720.

If there is at least one SSB/CSI-RS with SS-RSRP/CSI-RSRP above aconfigured ss-rsrp-threshold/csi-rsrp-threshold amongst the SSB(s)/CSIRS(s) associated with pre-allocated UL grant(s) that is available, theUE 1701 performs the following operation in step 1730:

-   -   UE skip RACH towards the target cell.        -   Select an SSB/CSI-RS with SS-RSRP/CSI-RSRP above a            configured ss-rsrp-threshold/csi-rsrp-threshold amongst the            SSB(s)/CSI RS(s) associated with pre-allocated UL grant(s)            (1740).            -   If multiple SSBs/CSI-RSs with SS-RSRP/CSI-RSRP above a                configured ss-rsrp-threshold/csi-rsrp-threshold are                available, UE select any one or UE select the SSB/CSI RS                whose associated UL grant is available first (in time                domain).        -   Transmit ReconfigurationComplete in UL grant (amongst the            pre-allocated UL grants) corresponding to selected            SSB/CSI-RS (1750).

If there is not any SSB/CSI-RS with an SS-RSRP/CSI-RSRP above aconfigured ss-rsrp-threshold/csi-rsrp-threshold amongst the SSB(s)/CSIRS(s) associated with the pre-allocated UL grant(s), the UE 1701performs the following operation in step 1730:

-   -   UE fallbacks to random access procedure i.e. it initiates random        access procedure (1760).        -   ReconfigurationComplete is transmitted in UL grant received            in RAR.

The ss-rsrp-threshold/csi-rsrp-threshold is signaled by the gNB 1703 inan RRCReconfiguration message. The ss-rsrp-threshold is configured if aUL grant(s) in a pre-allocated UL grant configuration is associated withSSBs. The csi-rsrp-threshold is configured if a UL grant(s) in apre-allocated UL grant configuration is associated with a CSI-RS(s). Thess-rsrp-threshold is configured in a RACH configuration.

The UE 1701 receives an RRCReconfiguration message from the gNB 1703 instep 1710. spCellConfig in the RRCReconfiguration message includesreconfigurationWithSync. Information (e.g. an indication to skip RACH)in reconfigurationWithSync IE indicates that the UE 1701 shall skip RACHtowards the target cell 1705. The RRCReconfiguration message includes apre-allocated UL grant configuration (parameters indicating periodicallyoccurring UL grants). This configuration is provided for at least a ULBWP indicated by firstActiveUplinkBWP-Id. The RRCReconfiguration messagealso includes information (i.e., SSB ID/CSI-RS ID) about theSSB/CSI-RS(s) associated with the pre-allocated UL grants.

The UE 1701 measures the SS-RSRP/CSI-RSRP of SSB/CSI-RS(s) associatedwith the pre-allocated UL grants in step 1720.

If there is at least one SSB/CSI-RS with an SS-RSRP/CSI-RSRP above aconfigured ss-rsrp-threshold/csi-rsrp-threshold amongst the SSB(s)/CSIRS(s) associated with the pre-allocated UL grant(s) that is available,the UE 1701 performs the following operation in 1730:

-   -   UE skip RACH towards the target cell        -   Select an SSB/CSI-RS with SS-RSRP/CSI-RSRP above a            configured ss-rsrp-threshold/csi-rsrp-threshold amongst the            SSB(s)/CSI RS(s) associated with pre-allocated UL grant(s)            (1740).            -   If multiple SSBs/CSI-RSs with SS-RSRP/CSI-RSRP above a                configured ss-rsrp-threshold/csi-rsrp-threshold are                available, UE select any one or UE select the SSB/CSI RS                whose associated UL grant is available first (in time                domain).        -   Transmit ReconfigurationComplete in UL grant (amongst the            pre-allocated UL grants) corresponding to selected            SSB/CSI-RS (1750).

If there is not any SSB/CSI-RS with an SS-RSRP/CSI-RSRP above aconfigured ss-rsrp-threshold/csi-rsrp-threshold amongst the SSB(s)/CSIRS(s) associated with the pre-allocated UL grant(s), the UE 1701performs the following operation in step 1730:

-   -   If fallback Indication is set to TRUE in RRCReconfiguration        message, UE fallbacks to random access procedure i.e. it        initiates random access procedure (1760).        -   ReconfigurationComplete is transmitted in UL grant received            in RAR.    -   Else:        -   Select any SSB/CSI-RS amongst the SSB(s)/CSI RS(s)            associated with pre-allocated UL grant(s).        -   Transmit ReconfigurationComplete in UL grant (amongst the            pre-allocated UL grants) corresponding to selected            SSB/CSI-RS.

The ss-rsrp-threshold/csi-rsrp-threshold is signaled by the gNB 1703 inthe RRCReconfiguration message. The ss-rsrp-threshold is configured if aUL grant(s) in a pre-allocated UL grant configuration is associated withSSBs. The csi-rsrp-threshold is configured if a UL grant(s) in apre-allocated UL grant configuration is associated with a CSI-RS(s). Thess-rsrp-threshold may be configured in a RACH configuration.

The UE 1701 receives an RRCReconfiguration message from the gNB 1703 instep 1710. spCellConfig in the RRCReconfiguration message includesreconfigurationWithSync. Information (e.g. an indication to skip RACH)in reconfigurationWithSync IE indicates that the UE 1701 shall skip RACHtowards the target cell 1705. The RRCReconfiguration message includes apre-allocated UL grant configuration (parameters indicating periodicallyoccurring UL grants). This configuration is provided for at least a ULBWP indicated by firstActiveUplinkBWP-Id. The RRCReconfiguration messagealso includes information (i.e., an SSB ID/CSI-RS ID) about theSSB/CSI-RS(s) associated with the pre-allocated UL grants.

The UE 1701 measures the SS-RSRP/CSI-RSRP of SSB/CSI-RS(s) associatedwith the pre-allocated UL grants in step 1720.

If there is at least one SSB/CSI-RS with an SS-RSRP/CSI-RSRP above aconfigured ss-rsrp-threshold/csi-rsrp-threshold amongst the SSB(s)/CSIRS(s) associated with the pre-allocated UL grant(s) that is available,the UE 1701 performs the following operation in step 1730:

-   -   UE skip RACH towards the target cell        -   Select an SSB/CSI-RS with SS-RSRP/CSI-RSRP above a            configured ss-rsrp-threshold/csi-rsrp-threshold amongst the            SSB(s)/CSI RS(s) associated with pre-allocated UL grant(s)            (1740).            -   If multiple SSBs/CSI-RSs with SS-RSRP/CSI-RSRP above a                configured ss-rsrp-threshold/csi-rsrp-threshold are                available, UE select any one or UE select the SSB/CSI RS                whose associated UL grant is available first (in time                domain).        -   Transmit ReconfigurationComplete in UL grant (amongst the            pre-allocated UL grants) corresponding to selected            SSB/CSI-RS (1750).

If there is not any SSB/CSI-RS with an SS-RSRP/CSI-RSRP above aconfigured ss-rsrp-threshold/csi-rsrp-threshold amongst the SSB(s)/CSIRS(s) associated with the pre-allocated UL grant(s), the UE 1701performs the following operation in step 1730:

-   -   If number of SSBs/CSI-RSs associated with pre-allocated UL grant        is less than the maximum SSBs/CSIRSs supported in target cell,        UE fallbacks to random access procedure i.e. it initiates random        access procedure.        -   ReconfigurationComplete is transmitted in UL grant received            in RAR.    -   Else:        -   Select any SSB/CSI-RS amongst the SSB(s)/CSI RS(s)            associated with pre-allocated UL grant(s).        -   Transmit ReconfigurationComplete in UL grant (amongst the            pre-allocated UL grants) corresponding to selected            SSB/CSI-RS (1760).

The ss-rsrp-threshold/csi-rsrp-threshold is signaled by the gNB 1703 inRRCReconfiguration message. The ss-rsrp-threshold is configured if theUL grant(s) in the pre-allocated UL grant configuration are associatedwith the SSBs. The csi-rsrp-threshold is configured if the UL grant(s)in the pre-allocated UL grant configuration are associated with theCSI-RS(s). The ss-rsrp-threshold may be configured in a RACHconfiguration.

FIG. 18 is a flowchart of a method of handling a fallback to randomaccess during a random access channel less (RACH-less) handoveraccording to an embodiment.

Referring to FIG. 18, the UE 1801 receives an RRCReconfiguration messagefrom a source cell 1803 (e.g. a gNB) in step 1810. spCellConfig in theRRCReconfiguration message includes reconfigurationWithSync. Information(e.g. an indication to skip RACH) in reconfigurationWithSync IEindicates that the UE 1801 shall skip RACH towards the target cell 1805.The RRCReconfiguration message includes a pre-allocated UL grantconfiguration (parameters indicate periodically occurring UL grants).This configuration is provided for at least a UL BWP indicated byfirstActiveUplinkBWP-Id. The RRCReconfiguration message also includesinformation (i.e., an SSB ID/CSI-RS ID) about the SSB/CSI-RS(s)associated with the pre-allocated UL grants in step 1820.

The UE 1801 starts the fallback timer in step 1830.

The UE 1801 measures the SS-RSRP/CSI-RSRP of SSB/CSI-RS(s) associatedwith the pre-allocated UL grants.

If the fallback timer is running and there is at least one SSB/CSI-RSwith an SS-RSRP/CSI-RSRP above a configuredss-rsrp-threshold/csi-rsrp-threshold amongst the SSB(s)/CSI RS(s)associated with pre-allocated UL grant(s) that is available, the UE 1801performs the following operation in step 1840:

-   -   UE stops the fallback timer (1860).    -   UE skip RACH towards the target cell.        -   Select an SSB/CSI-RS with SS-RSRP/CSI-RSRP above a            configured ss-rsrp-threshold/csi-rsrp-threshold amongst the            SSB(s)/CSI RS(s) associated with pre-allocated UL grant(s)            (1870).            -   If multiple SSBs/CSI-RSs with SS-RSRP/CSI-RSRP above a                configured ss-rsrp-threshold/csi-rsrp-threshold are                available, UE select any one or UE select the SSB/CSI RS                whose associated UL grant is available first (in time                domain).        -   Transmit ReconfigurationComplete in UL grant (amongst the            pre-allocated UL grants) corresponding to selected            SSB/CSI-RS (1880).

Measurement and comparison of measurements with threshold may beperformed periodically until the fallback timer expires or the UE 1801is able to select an SSB/CSI-RS above the threshold.

If the fallback timer expires, the UE 1801 fallbacks to a random accessprocedure, i.e., the UE 1801 initiates a random access procedure.ReconfigurationComplete is transmitted in the UL grant received in anRAR in step 1850.

The ss-rsrp-threshold/csi-rsrp-threshold is signaled by the gNB 1803 inthe RRCReconfiguration message. The ss-rsrp-threshold is configured ifthe UL grant(s) in the pre-allocated UL grant configuration areassociated with SSBs. The csi-rsrp-threshold is configured if the ULgrant(s) in the pre-allocated UL grant configuration are associated witha CSI-RS(s). The ss-rsrp-threshold may be configured in a RACHconfiguration.

Fallback timer duration is signaled by the gNB 1803 in theRRCReconfiguration message. The fallback timer duration may bepre-defined.

The UE 1801 receives the RRCReconfiguration message from the gNB 1803 instep 1810. spCellConfig in the RRCReconfiguration message includesreconfigurationWithSync. Information (e.g. an indication to skip RACH)in reconfigurationWithSync IE indicates that the UE 1801 shall skip RACHtowards the target cell 1805. The RRCReconfiguration message includes apre-allocated UL grant configuration (parameters indicate periodicallyoccurring UL grants). This configuration is provided for at least a ULBWP indicated by firstActiveUplinkBWP-Id. The RRCReconfiguration messagealso includes information (i.e., an SSB ID/CSI-RS ID) about anSSB/CSI-RS(s) associated with the pre-allocated UL grants in step 1820.

If the fallback timer is configured in the RRCReconfiguration message,the UE 1801 starts the fallback timer in step 1830.

The UE 1801 measures the SS-RSRP/CSI-RSRP of an SSB/CSI-RS(s) associatedwith the pre-allocated UL grants.

-   -   If the fallback timer is not configured in the        RRCReconfiguration message and if there is at least one        SSB/CSI-RS with an SS-RSRP/CSI-RSRP above a configured        ss-rsrp-threshold/csi-rsrp-threshold amongst the SSB(s)/CSI        RS(s) associated with the pre-allocated UL grant(s) that is        available, the UE 1801 performs the following operation in step        1840:        -   UE skip RACH towards the target cell.            -   Select an SSB/CSI-RS with SS-RSRP/CSI-RSRP above a                configured ss-rsrp-threshold/csi-rsrp-threshold amongst                the SSB(s)/CSI RS(s) associated with pre-allocated UL                grant(s) (1870).                -   If multiple SSBs/CSI-RSs with SS-RSRP/CSI-RSRP above                    a configured ss-rsrp-threshold/csi-rsrp-threshold                    are available, UE select any one or UE select the                    SSB/CSI RS whose associated UL grant is available                    first (in time domain).            -   Transmit ReconfigurationComplete in UL grant (amongst                the pre-allocated UL grants) corresponding to selected                SSB/CSI-RS (1880).    -   If there isn't any SSB/CSI-RS with SS-RSRP/CSI-RSRP above a        configured ss-rsrp-threshold/csi-rsrp-threshold, amongst the        SSB(s)/CSI RS(s) associated with pre-allocated UL grant(s), UE        performs the following operation:        -   UE fallbacks to random access procedure i.e. it initiates            random access procedure (1850).            -   ReconfigurationComplete is transmitted in UL grant                received in RAR.

Else, if the fallback timer is configured in the RRCReconfigurationmessage:

-   -   If fallback timer is running and if there is at least one        SSB/CSI-RS with SS-RSRP/CSI-RSRP above a configured        ss-rsrp-threshold/csi-rsrp-threshold, amongst the SSB(s)/CSI        RS(s) associated with pre-allocated UL grant(s), is available,        UE performs the following operation (1840):        -   UE stops the fallback timer (1860).        -   UE skip RACH towards the target cell.            -   Select an SSB/CSI-RS with SS-RSRP/CSI-RSRP above a                configured ss-rsrp-threshold/csi-rsrp-threshold amongst                the SSB(s)/CSI RS(s) associated with pre-allocated UL                grant(s) (1870).                -   If multiple SSBs/CSI-RSs with SS-RSRP/CSI-RSRP above                    a configured ss-rsrp-threshold/csi-rsrp-threshold                    are available, UE select any one or UE select the                    SSB/CSI RS whose associated UL grant is available                    first (in time domain).            -   Transmit ReconfigurationComplete in UL grant (amongst                the pre-allocated UL grants) corresponding to selected                SSB/CSI-RS (1880).    -   Measurement and comparison of measurements with threshold can be        performed periodically until fallback timer expires or UE is        able to select a SSB/CSI-RS above threshold.    -   If fallback timer expires, UE fallbacks to random access        procedure i.e. it initiates random access procedure.        ReconfigurationComplete is transmitted in UL grant received in        RAR (1850).

The ss-rsrp-threshold/csi-rsrp-threshold is configured as signaled bythe gNB 1803 in the RRCReconfiguration message. The ss-rsrp-threshold isconfigured if the UL grant(s) in the pre-allocated UL grantconfiguration is associated with SSBs. The csi-rsrp-threshold isconfigured if the UL grant(s) in the pre-allocated UL grantconfiguration is associated with a CSI-RS(s). The ss-rsrp-threshold maybe configured in a RACH configuration.

The UE 1801 receives a RRCReconfiguration message from the gNB 1803.spCellConfig in the RRCReconfiguration message includesreconfigurationWithSync. Information (e.g., an indication to skip RACH)in reconfigurationWithSync IE indicates that the UE 1801 shall skip RACHtowards the target cell 1805. The RRCReconfiguration message includes apre-allocated UL grant configuration (parameters indicating periodicallyoccurring UL grants). This configuration is provided for at least a ULBWP indicated by firstActiveUplinkBWP-Id. The RRCReconfiguration messagealso includes information (i.e., an SSB ID/CSI-RS ID) about theSSB/CSI-RS(s) associated with the pre-allocated UL grants.

If a fallback timer is configured in the RRCReconfiguration message, theUE 1801 starts the fallback timer.

The UE 1801 measures the SS-RSRP/CSI-RSRP of an SSB/CSI-RS(s) associatedwith the pre-allocated UL grants.

-   -   If there is at least one SSB/CSI-RS with SS-RSRP/CSI-RSRP above        a configured ss-rsrp-threshold/csi-rsrp-threshold, amongst the        SSB(s)/CSI RS(s) associated with pre-allocated UL grant(s), is        available, UE performs the following operation:        -   UE skip RACH towards the target cell            -   Select an SSB/CSI-RS with SS-RSRP/CSI-RSRP above a                configured ss-rsrp-threshold/csi-rsrp-threshold amongst                the SSB(s)/CSI RS(s) associated with pre-allocated UL                grant(s).                -   If multiple SSBs/CSI-RSs with SS-RSRP/CSI-RSRP above                    a configured ss-rsrp-threshold/csi-rsrp-threshold                    are available, UE select any one or UE select the                    SSB/CSI RS whose associated UL grant is available                    first (in time domain).            -   Transmit ReconfigurationComplete in UL grant (amongst                the pre-allocated UL grants) corresponding to selected                SSB/CSI-RS.    -   If there isn't any SSB/CSI-RS with SS-RSRP/CSI-RSRP above a        configured ss-rsrp-threshold/csi-rsrp-threshold, amongst the        SSB(s)/CSI RS(s) associated with pre-allocated UL grant(s), UE        performs the following operation:        -   If fallback indication is set to TRUE in RRCReconfiguration            message, UE fallbacks to random access procedure i.e. it            initiates random access procedure.            -   ReconfigurationComplete is transmitted in UL grant                received in RAR.    -   Else:        -   Select any SSB/CSI-RS amongst the SSB(s)/CSI RS(s)            associated with pre-allocated UL grant(s).        -   Transmit ReconfigurationComplete in UL grant (amongst the            pre-allocated UL grants) corresponding to selected            SSB/CSI-RS.

else if fallback timer is configured in RRCReconfiguration message:

-   -   If fallback timer is running and if there is at least one        SSB/CSI-RS with SS-RSRP/CSI-RSRP above a configured        ss-rsrp-threshold/csi-rsrp-threshold, amongst the SSB(s)/CSI        RS(s) associated with pre-allocated UL grant(s), is available,        UE performs the following operation:        -   UE stops the fallback timer        -   UE skip RACH towards the target cell            -   Select an SSB/CSI-RS with SS-RSRP/CSI-RSRP above a                configured ss-rsrp-threshold/csi-rsrp-threshold amongst                the SSB(s)/CSI RS(s) associated with pre-allocated UL                grant(s).                -   If multiple SSBs/CSI-RSs with SS-RSRP/CSI-RSRP above                    a configured ss-rsrp-threshold/csi-rsrp-threshold                    are available, UE select any one or UE select the                    SSB/CSI RS whose associated UL grant is available                    first (in time domain).            -   Transmit ReconfigurationComplete in UL grant (amongst                the pre-allocated UL grants) corresponding to selected                SSB/CSI-RS.    -   Measurement and comparison of measurements with threshold can be        performed periodically until fallback timer expires or UE is        able to select a SSB/CSI-RS above threshold.    -   If fallback timer expires, UE fallbacks to random access        procedure i.e. it initiates random access procedure.        ReconfigurationComplete is transmitted in UL grant received in        RAR.

The ss-rsrp-threshold/csi-rsrp-threshold is configured as signaled bythe gNB 1803 in the RRCReconfiguration message. The ss-rsrp-threshold isconfigured if the UL grant(s) in the pre-allocated UL grantconfiguration are associated with SSBs. The csi-rsrp-threshold isconfigured if the UL grant(s) in the pre-allocated UL grantconfiguration are associated with a CSI-RS(s). The ss-rsrp-threshold maybe configured in a RACH configuration.

The UE 1801 receives an RRCReconfiguration message from the gNB 1803.spCellConfig in the RRCReconfiguration message includesreconfigurationWithSync. Information (e.g., an indication to skip RACH)in reconfigurationWithSync IE indicates that the UE 1801 shall skip RACHtowards the target cell 1805. The RRCReconfiguration message includes apre-allocated UL grant configuration (parameters indicate periodicallyoccurring UL grants). This configuration is provided for at least a ULBWP indicated by firstActiveUplinkBWP-Id. The RRCReconfiguration messagealso includes information (i.e., an SSB ID/CSI-RS ID) about theSSB/CSI-RS(s) associated with the pre-allocated UL grants.

If the fallback timer is configured in the RRCReconfiguration message,the UE 1801 starts the fallback timer.

The UE 1801 measures the SS-RSRP/CSI-RSRP of an SSB/CSI-RS(s) associatedwith the pre-allocated UL grants.

If fallback timer is not configured in the RRCReconfiguration message:

-   -   If there is at least one SSB/CSI-RS with SS-RSRP/CSI-RSRP above        a configured ss-rsrp-threshold/csi-rsrp-threshold, amongst the        SSB(s)/CSI RS(s) associated with pre-allocated UL grant(s), is        available, UE performs the following operation:        -   UE skip RACH towards the target cell            -   Select an SSB/CSI-RS with SS-RSRP/CSI-RSRP above a                configured ss-rsrp-threshold/csi-rsrp-threshold amongst                the SSB(s)/CSI RS(s) associated with pre-allocated UL                grant(s).                -   If multiple SSBs/CSI-RSs with SS-RSRP/CSI-RSRP above                    a configured ss-rsrp-threshold/csi-rsrp-threshold                    are available, UE select any one or UE select the                    SSB/CSI RS whose associated UL grant is available                    first (in time domain).            -   Transmit ReconfigurationComplete in UL grant (amongst                the pre-allocated UL grants) corresponding to selected                SSB/CSI-RS.    -   If there isn't any SSB/CSI-RS with SS-RSRP/CSI-RSRP above a        configured ss-rsrp-threshold/csi-rsrp-threshold, amongst the        SSB(s)/CSI RS(s) associated with pre-allocated UL grant(s), UE        performs the following operation:        -   If number of SSBs/CSIRSs associated with pre-allocated UL            grant is less than the maximum SSBs/CSIRSs supported in            target cell, UE fallbacks to random access procedure i.e. it            initiates random access procedure.            -   ReconfigurationComplete is transmitted in UL grant                received in RAR.    -   Else:        -   Select any SSB/CSI-RS amongst the SSB(s)/CSI RS(s)            associated with pre-allocated UL grant(s).        -   Transmit ReconfigurationComplete in UL grant (amongst the            pre-allocated UL grants) corresponding to selected            SSB/CSI-RS.

else if fallback timer is configured in RRCReconfiguration message:

-   -   If fallback timer is running and if there is at least one        SSB/CSI-RS with SS-RSRP/CSI-RSRP above a configured        ss-rsrp-threshold/csi-rsrp-threshold, amongst the SSB(s)/CSI        RS(s) associated with pre-allocated UL grant(s), is available,        UE performs the following operation:        -   UE stops the fallback timer        -   UE skip RACH towards the target cell            -   Select an SSB/CSI-RS with SS-RSRP/CSI-RSRP above a                configured ss-rsrp-threshold/csi-rsrp-threshold amongst                the SSB(s)/CSI RS(s) associated with pre-allocated UL                grant(s).                -   If multiple SSBs/CSI-RSs with SS-RSRP/CSI-RSRP above                    a configured ss-rsrp-threshold/csi-rsrp-threshold                    are available, UE select any one or UE select the                    SSB/CSI RS whose associated UL grant is available                    first (in time domain).            -   Transmit ReconfigurationComplete in UL grant (amongst                the pre-allocated UL grants) corresponding to selected                SSB/CSI-RS.    -   Measurement and comparison of measurements with threshold can be        performed periodically until fallback timer expires or UE is        able to select a SSB/CSI-RS above threshold.    -   If fallback timer expires, UE fallbacks to random access        procedure i.e. it initiates random access procedure.        ReconfigurationComplete is transmitted in UL grant received in        RAR.

The ss-rsrp-threshold/csi-rsrp-threshold is configured as signaled bythe gNB 1803 in the RRCReconfiguration message. The ss-rsrp-threshold isconfigured if the UL grant(s) in the pre-allocated UL grantconfiguration is associated with SSBs. The csi-rsrp-threshold isconfigured if the UL grant(s) in the pre-allocated UL grantconfiguration is associated with a CSI-RS(s). The ss-rsrp-threshold maybe configured in a RACH configuration.

Fallback based on TAT timer

FIG. 19 is an illustration of handling a fallback to random accessduring a random access channel less (RACH-less) handover according to anembodiment of the disclosure,

Referring to FIG. 19, a handover command indicates a RACH-less handover.The handover command indicates to use TA of a source cell for ULtransmission in a target cell. A TAT timer remaining in the source cellat the time the handover command is received is X ms (e.g. 10 ms). TheTA of the source cell may become invalid during handover execution ifRACH-less handover is not completed within X ms. If T is greater than X,the TA value will become invalid.

Various methods of overcoming the above-mentioned issue are describedbelow:

A UE receives an RRCReconfiguration message from a gNB. spCellConfig inthe RRCReconfiguration message includes reconfigurationWithSync.Information (e.g. an indication to skip RACH) in reconfigurationWithSyncIE indicates that the UE shall skip RACH towards the target cell. Theinformation also indicates that the target timing advance (i.e., atargetTA) (i.e., a TA of a primary timing advance group (PTAG) or asecondary timing advance group (STAG) in the source gNB), that the UEshould use for a UL transmission in the PTAG at the target gNB. TheRRCReconfiguration message includes a pre-allocated UL grantconfiguration (parameters indicate periodically occurring UL grants).This configuration is provided for at least a UL BWP indicated byfirstActiveUplinkBWP-Id.

The UE initializes the TAT timer of the PTAG to a remaining value of TATtimer of the targetTA.

The UE starts using the targetTA (i.e., the TA of the PTAG or the STAGin the source gNB) for the UL transmission in the target cell.

If the TAT timer expires and if the RACH-less handover is not completed(e.g., the UE has not received the MAC CE indicating confirmation ofcompletion of the RACH-less handover):

-   -   The UE falls back to a random access procedure i.e., the UE        initiates a random access procedure.

Alternatively, if the TAT timer expires and if transmission on thepre-allocated UL grant is not yet successful,

-   -   the UE falls back to random access procedure, i.e., initiates        random access procedure.

The UE receives the RRCReconfiguration message from the gNB.spCellConfig in the RRCReconfiguration message includesreconfigurationWithSync. Information (e.g. an indication to skip RACH)in reconfigurationWithSync IE indicates that the UE shall skip RACHtowards the target cell. The information also indicates the targetTA(i.e., the TA of the PTAG or the STAG in the source gNB) that the UEshould use for a UL transmission in that PTAG at the target gNB. TheRRCReconfiguration message includes pre-allocated UL grant configuration(parameters indicate periodically occurring UL grants). Thisconfiguration is provided for at least a UL BWP indicated byfirstActiveUplinkBWP-Id.

The UE starts a fallback timer. The fallback timer is set to theremaining value of the TAT timer of the targetTA.

The UE starts the timeAlignmentTimer associated with the PTAG.

The UE starts using the targetTA (i.e., the TA of the PTAG or the STAGin the source gNB) for the UL transmission in the target cell.

If the fallback timer expires and if the RACH-less handover is notcompleted (e.g., the UE has not received the MAC CE indicatingconfirmation of completion of the RACH-less handover):

-   -   the UE falls back to a random access procedure, i.e., initiates        random access procedure

Alternatively, if the fallback timer expires and if transmission on thepre-allocated UL grant is not yet successful,

-   -   the UE falls back to random access procedure i.e., the UE        initiates a random access procedure.

The UL grant may either be pre-configured in the RRCReconfigurationmessage or if not pre-configured then provided on the PDCCH in thetarget cell.

Regardless of the option used by the network to provide a UL grant, thevalidity of the UL grant may be based on a timer (specified in number ofradio frames) provided in RRCReconfiguration message.

-   -   If such a timer is configured, then the start of the timer at        the UE is a system frame number (SFN) in which it receives the        RRCReconfiguration message.

The UE shall fallback to random access on a target gNB:

-   -   a. If transmission on pre-configured UL grants is not successful        until expiration of the TAT (if running) having reference to any        serving cell of the source gNB or expiration of the UL grant        validity timer if configured (whichever expires first), or    -   b. If none of the gNB beams (SSBs/CSI RSs) associated with        pre-allocated UL grants are suitable.

If the UE fallbacks to random access on the target gNB, then CBRA on thetarget gNB continue until expiration of T304.

If a validity timer is not configured in the RRCReconfiguration messagethen pre-configured UL grants are valid until expiration of T304

FIG. 20 is a block diagram of a terminal 2000 according to anembodiment.

Referring to FIG. 20, the terminal 2000 includes a transceiver 2010, acontroller 2020 and a memory 2030. The controller 2020 may refer tocircuitry, an ASIC, or at least one processor. The transceiver 2010, thecontroller 2020 and the memory 2030 are configured to perform theoperations of the UE illustrated in FIGS. 1 to 19, or described above.Although the transceiver 2010, the controller 2020 and the memory 2030are shown as separate entities, they may be realized as a single entitylike a single integrated circuit or chip. The transceiver 2010, thecontroller 2020 and the memory 2030 may be electrically connected to orcoupled with each other.

The transceiver 2010 may transmit and receive signals to and from othernetwork entities, e.g., a base station.

The controller 2020 may control the UE to perform functions according toone of the embodiments described above. The operations of the terminal2000 may be implemented using the memory 2030 storing correspondingprogram code. Specifically, the terminal 2000 may be equipped with thememory 2030 to store program code implementing desired operations. Toperform the desired operations, the controller 2020 may read and executethe program code stored in the memory 2030 by using a processor or acentral processing unit (CPU). Alternatively, the controller 2020 may beimplemented as at least one processor.

FIG. 21 is a block diagram of a base station 2100 according to anembodiment.

Referring to FIG. 21, the base station 2100 includes a transceiver 2110,a controller 2120 and a memory 2130. The transceiver 2110, thecontroller 2120 and the memory 2130 are configured to perform theoperations of the network (e.g., an gNB) illustrated in FIGS. 1 to 19,or described above. Although the transceiver 2110, the controller 2120and the memory 2130 are shown as separate entities, they may be realizedas a single entity like a single chip. The transceiver 2110, thecontroller 2120 and the memory 2130 may be electrically connected to orcoupled with each other.

The transceiver 2110 may transmit and receive signals to and from othernetwork entities, e.g., a terminal. The controller 2120 may control thebase station 2100 to perform functions according to one of theembodiments described above. The controller 2120 may refer to circuitry,an ASIC, or at least one processor. The operations of the base station2100 may be implemented using the memory 2130 storing correspondingprogram code. Specifically, the base station 2100 may be equipped withthe memory 2130 to store program code implementing desired operations.To perform the desired operations, the controller 2120 may read andexecute the program code stored in the memory 2130 by using a processoror a CPU. Alternatively, the controller 2120 may be implemented as atleast one processor.

While the disclosure has been shown and described with reference tovarious embodiments thereof, it will be understood by those skilled inthe art that various changes in form and details may be made thereinwithout departing from the spirit and scope of the disclosure as definedby the appended claims and their equivalents.

What is claimed is:
 1. A method performed by a terminal in a wirelesscommunication system, the method comprising: storing a medium accesscontrol (MAC) protocol data unit (PDU) in a buffer, the MAC PDU beingobtained for a message A (MSG A) transmission; transmitting, to a basestation, the MSG A including the MAC PDU; receiving, from the basestation, a message B (MSG B) as a response to the MSG A, the MSG Bincluding fallback information; and transmitting, to the base station,an uplink transmission based on the fallback information, the uplinktransmission including the MAC PDU.
 2. The method of claim 1, whereinthe uplink transmission includes a message 3 (MSG 3) transmission. 3.The method of claim 1, wherein the uplink transmission is transmittedbased on the fallback information, in case that a random access preambleidentifier (RAPID) included in the fallback information matches an indexof a random access preamble transmitted in the MSG A.
 4. The method ofclaim 1, wherein the MSG A includes a physical random access channel(PRACH) transmission and a physical uplink shared channel (PUSCH)transmission, wherein the MSG A is transmitted based on a controlmessage including information on a PUSCH resource for the MSG A, andwherein the information on the PUSCH resource includes at least one ofinformation on a resource block (RB) offset, information on a number ofRBs, information on a number of frequency division multiplexed PUSCHoccasions, information on a slot offset, or information on a startingsymbol.
 5. The method of claim 1, wherein the MSG B is received bymonitoring a window starting at a first physical downlink controlchannel (PDCCH) monitoring occasion which is at least one symbol after alast symbol of a PUSCH transmission of the MSG A.
 6. A method performedby a base station in a wireless communication system, the methodcomprising: receiving, from a terminal, a message A (MSG A) including amedium access control (MAC) protocol data unit (PDU), wherein the MACPDU is obtained by the terminal for the MSG A transmission and is storedin a buffer of the terminal; transmitting, to the terminal, a message B(MSG B) as a response to the MSG A, the MSG B including fallbackinformation; and receiving, from the terminal, an uplink transmissionbased on the fallback information, the uplink transmission including theMAC PDU.
 7. The method of claim 6, wherein the uplink transmissionincludes a message 3 (MSG 3) transmission.
 8. The method of claim 6,wherein the uplink transmission is received based on the fallbackinformation, in case that a random access preamble identifier (RAPID)included in the fallback information matches an index of a random accesspreamble received in the MSG A.
 9. The method of claim 6, wherein theMSG A includes a physical random access channel (PRACH) transmission anda physical uplink shared channel (PUSCH) transmission, wherein the MSG Ais received based on a control message including information on a PUSCHresource for the MSG A, and wherein the information on the PUSCHresource includes at least one of information on a resource block (RB)offset, information on a number of RBs, information on a number offrequency division multiplexed PUSCH occasions, information on a slotoffset, or information on a starting symbol.
 10. The method of claim 6,wherein the MSG B is received by the terminal, by monitoring a windowstarting at a first physical downlink control channel (PDCCH) monitoringoccasion which is at least one symbol after a last symbol of a PUSCHtransmission of the MSG A.
 11. A terminal in a wireless communicationsystem, the terminal comprising: a transceiver; and a controllerconfigured to: store a medium access control (MAC) protocol data unit(PDU) in a buffer, the MAC PDU being obtained for a message A (MSG A)transmission, transmit, to a base station via the transceiver, the MSG Aincluding the MAC PDU, receive, from the base station via thetransceiver, a message B (MSG B) as a response to the MSG A, the MSG Bincluding fallback information, and transmit, to the base station viathe transceiver, an uplink transmission based on the fallbackinformation, the uplink transmission including the MAC PDU.
 12. Theterminal of claim 11, wherein the uplink transmission includes a message3 (MSG 3) transmission.
 13. The terminal of claim 11, wherein the uplinktransmission is transmitted based on the fallback information, in casethat a random access preamble identifier (RAPID) included in thefallback information matches an index of a random access preambletransmitted in the MSG A.
 14. The terminal of claim 11, wherein the MSGA includes a physical random access channel (PRACH) transmission and aphysical uplink shared channel (PUSCH) transmission, wherein the MSG Ais transmitted based on a control message including information on aPUSCH resource for the MSG A, and wherein the information on the PUSCHresource includes at least one of information on a resource block (RB)offset, information on a number of RBs, information on a number offrequency division multiplexed PUSCH occasions, information on a slotoffset, or information on a starting symbol.
 15. The terminal of claim11, wherein the MSG B is received by monitoring a window starting at afirst physical downlink control channel (PDCCH) monitoring occasionwhich is at least one symbol after a last symbol of a PUSCH transmissionof the MSG A.
 16. A base station in a wireless communication system, thebase station comprising: a transceiver; and a controller configured to:receive, from a terminal via the transceiver, a message A (MSG A)including a medium access control (MAC) protocol data unit (PDU),wherein the MAC PDU is obtained by the terminal for the MSG Atransmission and is stored in a buffer of the terminal, transmit, to theterminal via the transceiver, a message B (MSG B) as a response to theMSG A, the MSG B including fallback information, and receive, from theterminal via the transceiver, an uplink transmission based on thefallback information, the uplink transmission including the MAC PDU. 17.The base station of claim 16, wherein the uplink transmission includes amessage 3 (MSG 3) transmission.
 18. The base station of claim 16,wherein the uplink transmission is received based on the fallbackinformation, in case that a random access preamble identifier (RAPID)included in the fallback information matches an index of a random accesspreamble received in the MSG A.
 19. The base station of claim 16,wherein the MSG A includes a physical random access channel (PRACH)transmission and a physical uplink shared channel (PUSCH) transmission,wherein the MSG A is received based on a control message includinginformation on a PUSCH resource for the MSG A, and wherein theinformation on the PUSCH resource includes at least one of informationon a resource block (RB) offset, information on a number of RBs,information on a number of frequency division multiplexed PUSCHoccasions, information on a slot offset, or information on a startingsymbol.
 20. The base station of claim 16, wherein the MSG B is receivedby the terminal, by monitoring a window starting at a first physicaldownlink control channel (PDCCH) monitoring occasion which is at leastone symbol after a last symbol of a PUSCH transmission of the MSG A.